Inclusion Criteria:

          -  Patients must have histologically documented unresectable locally advanced or
             metastatic non-HER2-positive breast cancer and a known BRCA 1/2 mutation present; both
             germline and somatic mutations are acceptable, however somatic mutations must be
             identified by tumor sequencing and not blood; patients with BRCA mutations of unknown
             significance are not allowed

          -  Patients must have measurable disease, defined as at least one lesion that can be
             accurately measured in at least one dimension (longest diameter to be recorded for
             non-nodal lesions and short axis for nodal lesions) as >= 20 mm (>= 2 cm) by chest
             x-ray or as >= 10 mm (>= 1 cm) with computed tomography (CT) scan, magnetic resonance
             imaging (MRI), or calipers by clinical exam

          -  Prior chemotherapy is allowed, including platinum therapy; patients must not have
             received chemotherapy for 4 weeks prior to the initiation of study treatment and must
             have recovery =< grade 1 from any adverse events from any prior chemotherapy (other
             than alopecia); patients must not have had nitrosoureas or mitomycin C for 6 weeks
             prior to the initiation of study treatment

          -  Prior radiation therapy is allowed; patients must not have received minimal radiation
             therapy (=< 5% of their total marrow volume) within 3 weeks prior to the initiation of
             study treatment; otherwise, patients must not have received radiation therapy (> 5% of
             their total marrow volume) within 4 weeks prior to the initiation of study treatment;
             patients who have received prior radiation to 50% or more of their total marrow volume
             will be excluded

          -  Patients who have received prior treatment with anti-CTLA-4 may be enrolled, provided
             the following requirements are met: minimum of 12 weeks from the first dose of
             anti-CTLA-4 and > 6 weeks from the last dose, and no history of severe immune-related
             adverse effects from anti-CTLA-4 (National Cancer Institute [NCI] Common Terminology
             Criteria for Adverse Events [CTCAE] grade 3 and 4)

          -  Treatment with systemic immunosuppressive medications (including, but not limited to,
             prednisone, cyclophosphamide, azathioprine, methotrexate, thalidomide, and anti-tumor
             necrosis factor [anti-TNF] agents) is allowed, provided the following is met: minimum
             of 2 weeks prior to cycle 1, day 1; patients who have received acute, low dose,
             systemic immunosuppressant medications (e.g., a one-time dose of dexamethasone for
             nausea) may be enrolled; the use of corticosteroids and mineralocorticoids (e.g.,
             fludrocortisone) for patients with orthostatic hypotension or adrenocortical
             insufficiency is allowed

          -  Prior treatment with systemic immunostimulatory agents (including, but not limited to,
             interferon [IFN]-alpha or interleukin [IL]-2) is allowed, provided the following is
             met: minimum of 6 weeks prior to cycle 1, day 1

          -  Patients taking bisphosphonate therapy for symptomatic hypercalcemia are NOT allowed;
             use of bisphosphonate therapy for other reasons (e.g., bone metastasis or
             osteoporosis) is allowed

          -  Prior hormone therapy is allowed; patients must not have received hormone therapy for
             breast cancer for 2 weeks prior to the initiation of study treatment and must have
             recovery =< grade 1 from any adverse events related to these therapies (other than
             alopecia)

          -  Prior experimental (non-Food and Drug Administration [FDA] approved) therapies and
             immunotherapies are allowed; patients must not have received these therapies for 4
             weeks prior to the initiation of study treatment and must have recovery =< grade 1
             from any adverse events of these therapies (other than alopecia); prior treatment with
             any PARP inhibitor or any anti-PD-1/anti-PD-L1 antibody is NOT allowed

          -  Other therapies (e.g. targeted therapy such as cyclin-dependent kinase [CDK]
             inhibitors): patients should have recovered to =< grade 1 drug related toxicity; they
             must have completed therapy for either a total of duration equivalent to 5 half-lives
             of the drug or 28 days, whichever is shorter

          -  Eastern Cooperative Oncology Group (ECOG) performance status =< 2 (Karnofsky >= 60%)

          -  Life expectancy of greater than 6 months

          -  Absolute neutrophil count >= 1,500/mcL

          -  Leukocytes >= 3,000/mcL

          -  Platelets >= 100,000/mcL

          -  Hemoglobin >= 8 g/dL

          -  Total bilirubin =< 1.5 x upper limit of normal (ULN) (however, patients with known
             Gilbert disease who have serum bilirubin level =< 3 x ULN may be enrolled)

          -  Aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase
             [SGOT])/alanine aminotransferase (ALT) (serum glutamic pyruvic transaminase [SGPT]) =<
             2.5 x upper limit of normal (ULN) if no liver metastasis; =< 5 x upper ULN if liver
             metastasis present

          -  Alkaline phosphatase =< 2.5 x ULN (=< 5 x ULN for patients with documented liver
             involvement or bone metastases)

          -  Creatinine clearance >= 51 mL/min/1.73 m^2 by Cockcroft-Gault

          -  International normalized ratio (INR) and activated partial thromboplastin time (aPTT)
             =< 1.5 x ULN (this applies only to patients who do not receive therapeutic
             anticoagulation; patients receiving therapeutic anticoagulation, such as
             low-molecular-weight heparin or warfarin, should be on a stable dose)

          -  No features suggestive of myelodysplastic syndrome (MDS)/acute myeloid leukemia (AML)
             on peripheral blood smear when performed as clinically indicated

          -  Patients must have tumors determined to be easily accessible for biopsy and must be
             willing to have serial biopsies (with a third biopsy upon evidence of disease
             progression)

          -  Administration of atezolizumab and/or olaparib may have an adverse effect on pregnancy
             and poses a risk to the human fetus, including embryo-lethality; women of
             child-bearing potential and men must agree to use highly effective contraception prior
             to study entry, for the duration of study participation, and for at least 5 months
             (150 days) after the last dose of study agent; should a woman become pregnant or
             suspect she is pregnant while she or her partner is participating in this study, she
             should inform her treating physician immediately; men treated or enrolled on this
             protocol must also agree to use two highly effective forms of contraception in
             combination prior to the study, for the duration of study participation, and for at
             least 5 months (150 days) after completion of atezolizumab and/or olaparib
             administration; women of child-bearing potential: negative serum pregnancy test within
             28 days of study treatment and confirmed prior to treatment on day 1; postmenopausal
             or evidence of non-childbearing status for women of childbearing potential;
             postmenopausal is defined as:

               -  Amenorrheic for 1 year or more following cessation of exogenous hormonal
                  treatments

               -  Luteinizing hormone (LH) and follicle stimulating hormone (FSH) levels in the
                  post-menopausal range for women under 50

               -  Radiation-induced oophorectomy with last menses > 1 year ago

               -  Chemotherapy-induced menopause with > 1 year interval since last menses

               -  Surgical sterilization (bilateral oophorectomy or hysterectomy)

          -  Ability to understand and the willingness to sign a written informed consent document

          -  Subject is able to swallow and retain oral medication and does not have uncontrolled
             emesis or gastrointestinal disorders likely to interfere with absorption of the study
             medication

          -  Patients crossing over from monotherapy to combination therapy do not have to be fully
             rescreened; however, they do need to meet performance status, organ function, and
             blood parameters and not meet any of the exclusion criteria

          -  Patients positive for human immunodeficiency virus (HIV) are NOT excluded from this
             study, but HIV-positive patients must have:

               -  A stable regimen of highly active anti-retroviral therapy (HAART)

               -  No requirement for concurrent antibiotics or antifungal agents for the prevention
                  of opportunistic infections

               -  A CD4 count above 250 cells/mcL and an undetectable HIV viral load on standard
                  polymerase chain reaction (PCR)-based test

        Exclusion Criteria:

          -  Patients with prior allogeneic bone marrow transplantation, double umbilical cord
             blood transplantation (dUCBT) or prior solid organ transplantation

          -  Patients with known brain metastases should be excluded from this clinical trial
             except as those described below, because of their poor prognosis and because they
             often develop progressive neurologic dysfunction that would confound the evaluation of
             neurologic and other adverse events

          -  Patients with known primary central nervous system (CNS) malignancy or symptomatic CNS
             metastases are excluded, with the following exceptions:

               -  Patients with asymptomatic untreated CNS disease may be enrolled, provided all of
                  the following criteria are met:

                    -  Evaluable or measurable disease outside the CNS

                    -  No metastases to brain stem, midbrain, pons, medulla, cerebellum, or within
                       10 mm of the optic apparatus (optic nerves and chiasm)

                    -  No history of intracranial hemorrhage unless it is confined within a lesion
                       previously noted and secondary to gamma knife or another equivalent
                       radiologic therapeutic

                    -  No history of spinal cord hemorrhage

                    -  No ongoing requirement for dexamethasone for CNS disease; patients on a
                       stable dose of anticonvulsants are permitted

                    -  No neurosurgical resection or brain biopsy within 28 days prior to cycle 1,
                       day 1

               -  Patients with asymptomatic treated CNS metastases may be enrolled, provided all
                  the criteria listed above are met as well as the following:

                    -  Radiographic demonstration of improvement upon the completion of CNS
                       directed therapy and no evidence of interim progression between the
                       completion of CNS directed therapy and the screening radiographic study

                    -  No stereotactic radiation or whole-brain radiation within 28 days prior to
                       cycle 1, day 1

                    -  Screening CNS radiographic study >= 4 weeks from completion of radiotherapy
                       and >= 2 weeks from discontinuation of corticosteroids

          -  Known hypersensitivity to Chinese hamster ovary cell products or other recombinant
             human antibodies

          -  History of severe allergic, anaphylactic, or other hypersensitivity reactions to
             chimeric or humanized antibodies or fusion proteins

          -  History of allergic reactions attributed to compounds of similar chemical or biologic
             composition to olaparib and atezolizumab; patients with a known hypersensitivity to
             olaparib or any of the excipients of the product

          -  Prior treatment with any PARP inhibitor or any anti-PD-1/anti-PD-L1 antibody

          -  Known clinically significant liver disease, including active viral, alcoholic, or
             other hepatitis; cirrhosis; fatty liver; and inherited liver disease

               -  Patients with past or resolved hepatitis B infection (defined as having a
                  negative hepatitis B surface antigen [HBsAg] test and a positive anti-HBc
                  [antibody to hepatitis B core antigen] antibody test) are eligible

               -  Patients positive for hepatitis C virus (HCV) antibody are eligible only if PCR
                  is negative for HCV RNA

          -  History or risk of autoimmune disease, including, but not limited to, systemic lupus
             erythematosus, rheumatoid arthritis, inflammatory bowel disease, vascular thrombosis
             associated with antiphospholipid syndrome, Wegener's granulomatosis, Sjogren's
             syndrome, Bell's palsy, Guillain-Barre syndrome, multiple sclerosis, autoimmune
             thyroid disease, vasculitis, lichen sclerosis, or glomerulonephritis

               -  Patients with a history of autoimmune hypothyroidism on a stable dose of thyroid
                  replacement hormone may be eligible

               -  Patients with controlled type 1 diabetes mellitus on a stable insulin regimen may
                  be eligible

               -  Patients with eczema, psoriasis, lichen simplex chronicus of vitiligo with
                  dermatologic manifestations only (e.g., patients with psoriatic arthritis would
                  be excluded) are permitted provided that they meet the following conditions:

                    -  Patients with psoriasis must have a baseline ophthalmologic exam to rule out
                       ocular manifestations

                    -  Rash must cover less than 10% of body surface area (BSA)

                    -  Disease is well controlled at baseline and only requiring low potency
                       topical steroids (e.g., hydrocortisone 2.5%, hydrocortisone butyrate 0.1%,
                       fluocinolone 0.01%, desonide 0.05%, alclometasone dipropionate 0.05%)

                    -  No acute exacerbations of underlying condition within the last 12 months
                       (not requiring psoralen plus ultraviolet A radiation [PUVA], methotrexate,
                       retinoids, biologic agents, oral calcineurin inhibitors; high potency or
                       oral steroids)

          -  History of idiopathic pulmonary fibrosis, pneumonitis (including drug induced),
             organizing pneumonia (i.e., bronchiolitis obliterans, cryptogenic organizing
             pneumonia, etc.), or evidence of active pneumonitis on screening chest computed
             tomography (CT) scan; history of radiation pneumonitis in the radiation field
             (fibrosis) is permitted if recovered

          -  Major surgical procedure within 28 days prior to cycle 1, day 1 and patients must have
             recovered from any effects of any major surgery; anticipation of need for a major
             surgical procedure during the course of the study

          -  Administration of a live, attenuated vaccine within 4 weeks before cycle 1, day 1 or
             anticipation that such a live, attenuated vaccine will be required during the study
             and up to 5 months after the last dose of atezolizumab

               -  Influenza vaccination should be given during influenza season only (approximately
                  October to March); patients must not receive live, attenuated influenza vaccine
                  within 4 weeks prior to cycle 1, day 1 or at any time during the study

          -  Patients considered a poor medical risk due to a serious, uncontrolled medical
             disorder, non-malignant systemic disease or active, uncontrolled infection; examples
             include, but are not limited to, uncontrolled ventricular arrhythmia, recent (within 3
             months) myocardial infarction, unstable spinal cord compression, superior vena cava
             syndrome, symptomatic congestive heart failure, unstable angina pectoris, extensive
             interstitial bilateral lung disease on high resolution computed tomography (HRCT) scan
             or any psychiatric disorder that prohibits obtaining informed consent and would limit
             compliance with study requirements

          -  Pregnant women are excluded from this study because olaparib and atezolizumab are have
             the potential for teratogenic or abortifacient effects; because there is an unknown
             but potential risk for adverse events in nursing infants secondary to treatment of the
             mother with olaparib and atezolizumab, breastfeeding should be discontinued if the
             mother is treated with olaparib and atezolizumab

          -  Any prior grade >= 3 immune-related adverse event (irAE) while receiving any previous
             immunotherapy agent, or any unresolved irAE > grade 1

          -  Patients with active seizures or a history of uncontrolled seizure disorder, including
             focal or generalized seizure within the past year

          -  Patients requiring treatment with a RANKL inhibitor (e.g. denosumab) who cannot
             discontinue it before treatment with atezolizumab

          -  Resting electrocardiogram (ECG) with corrected QT (QTc) > 470