Inclusion Criteria:

          -  Patient must have enrolled onto APEC1621SC and must have been given a treatment
             assignment to Molecular Analysis for Therapy Choice (MATCH) to APEC1621H based on the
             presence of an actionable mutation

          -  Patients must have a body surface area >= 0.65 m^2 at enrollment

          -  Patients must have radiographically measurable disease at the time of study
             enrollment; patients with neuroblastoma who do not have measurable disease but have
             iobenguane (MIBG) positive (+) evaluable disease are eligible; measurable disease in
             patients with CNS involvement is defined as tumor that is measurable in two
             perpendicular diameters on magnetic resonance imaging (MRI) and visible on more than
             one slice

               -  Note: The following do not qualify as measurable disease:

                    -  Malignant fluid collections (e.g., ascites, pleural effusions)

                    -  Bone marrow infiltration except that detected by MIBG scan for neuroblastoma

                    -  Lesions only detected by nuclear medicine studies (e.g., bone, gallium or
                       positron emission tomography [PET] scans) except as noted for neuroblastoma

                    -  Elevated tumor markers in plasma or cerebrospinal fluid (CSF)

                    -  Previously radiated lesions that have not demonstrated clear progression
                       post radiation

                    -  Leptomeningeal lesions that do not meet the measurement requirements for
                       Response Evaluation Criteria in Solid Tumors (RECIST) 1.1

          -  Karnofsky >= 50% for patients > 16 years of age and Lansky >= 50 for patients =< 16
             years of age

               -  Note: Neurologic deficits in patients with CNS tumors must have been relatively
                  stable for at least 7 days prior to study enrollment; patients who are unable to
                  walk because of paralysis, but who are up in a wheelchair, will be considered
                  ambulatory for the purpose of assessing the performance score

          -  Patients must have fully recovered from the acute toxic effects of all prior
             anti-cancer therapy and must meet the following minimum duration from prior
             anti-cancer directed therapy prior to enrollment; if after the required timeframe, the
             numerical eligibility criteria are met, e.g. blood count criteria, the patient is
             considered to have recovered adequately

               -  Cytotoxic chemotherapy or other anti-cancer agents known to be myelosuppressive

                    -  >= 21 days after the last dose of cytotoxic or myelosuppressive chemotherapy
                       (42 days if prior nitrosourea)

               -  Anti-cancer agents not known to be myelosuppressive (e.g. not associated with
                  reduced platelet or absolute neutrophil count [ANC] counts): >= 7 days after the
                  last dose of agent

               -  Antibodies: >= 21 days must have elapsed from infusion of last dose of antibody,
                  and toxicity related to prior antibody therapy must be recovered to grade =< 1

               -  Corticosteroids: If used to modify immune adverse events related to prior
                  therapy, >= 14 days must have elapsed since last dose of corticosteroid

               -  Hematopoietic growth factors: >= 14 days after the last dose of a long-acting
                  growth factor (e.g. pegfilgrastim) or 7 days for short-acting growth factor; for
                  growth factors that have known adverse events occurring beyond 7 days after
                  administration, this period must be extended beyond the time during which adverse
                  events are known to occur; the duration of this interval must be discussed with
                  the study chair and the study-assigned research coordinator

               -  Interleukins, interferons and cytokines (other than hematopoietic growth
                  factors): >= 21 days after the completion of interleukins, interferon or
                  cytokines (other than hematopoietic growth factors)

               -  Stem cell infusions (with or without total body irradiation [TBI]):

                    -  Allogeneic (non-autologous) bone marrow or stem cell transplant, or any stem
                       cell infusion including donor lymphocyte infusion (DLI) or boost infusion:
                       >= 84 days after infusion and no evidence of graft versus host disease
                       (GVHD)

                    -  Autologous stem cell infusion including boost infusion: >= 42 days

               -  Cellular therapy: >= 42 days after the completion of any type of cellular therapy
                  (e.g. modified T cells, natural killer [NK] cells, dendritic cells, etc.)

               -  Radiation therapy (XRT)/external beam irradiation including protons: >= 14 days
                  after local XRT; >= 150 days after TBI, craniospinal XRT or if radiation to >=
                  50% of the pelvis; >= 42 days if other substantial bone marrow (BM) radiation

                    -  Note: Radiation may not be delivered to "measurable disease" tumor site(s)
                       being used to follow response to subprotocol treatment

               -  Radiopharmaceutical therapy (e.g., radiolabeled antibody, 131iodine [I]-MIBG): >=
                  42 days after systemically administered radiopharmaceutical therapy

               -  Patients must not have received prior exposure to olaparib, veliparib, niraparib,
                  rucaparib, talazoparib or other poly adenosine diphosphate ribose polymerase
                  inhibitors (PARPi)

          -  For patients with solid tumors without known bone marrow involvement: peripheral
             absolute neutrophil count (ANC) >= 1000/mm^3 (within 7 days prior to enrollment)

          -  For patients with solid tumors without known bone marrow involvement: platelet count
             >= 100,000/mm^3 (within 7 days prior to enrollment) (transfusion independent, defined
             as not receiving platelet transfusions for at least 7 days prior to enrollment)

          -  Patients with known bone marrow metastatic disease will be eligible for study provided
             they meet the blood counts (may receive platelet or packed red blood cells [pRBC]
             transfusions provided they are not known to be refractory to red cell or platelet
             transfusions); these patients will not be evaluable for hematologic toxicity

          -  Creatinine clearance or radioisotope glomerular filtration rate (GFR) >= 70
             ml/min/1.73 m^2 (within 7 days prior to enrollment) or

          -  A serum creatinine based on age/gender as follows (within 7 days prior to enrollment):

               -  Age 1 to < 2 years: maximum serum creatinine 0.6 mg/dL for male and 0.6 mg/dL for
                  female

               -  Age 2 to < 6 years: maximum serum creatinine 0.8 mg/dL for male and 0.8 mg/dL for
                  female

               -  Age 6 to < 10 years: maximum serum creatinine 1 mg/dL for male and 1 mg/dL for
                  female

               -  Age 10 to < 13 years: maximum serum creatinine 1.2 mg/dL for male and 1.2 mg/dL
                  for female

               -  Age 13 to < 16 years: maximum serum creatinine 1.5 mg/dL for male and 1.4 mg/dL
                  for female

               -  Age >= 16 years: maximum serum creatinine 1.7 mg/dL for male and 1.4 mg/dL for
                  female

          -  Patients with solid tumors: bilirubin (sum of conjugated + unconjugated) =< 1.5 x
             upper limit of normal (ULN) for age (within 7 days prior to enrollment)

          -  Patients with solid tumors: serum glutamic pyruvic transaminase (SGPT) (alanine
             aminotransferase [ALT]) =< 135 U/L (within 7 days prior to enrollment); (for the
             purpose of this study, the ULN for SGPT is 45 U/L)

          -  Patients with solid tumors: serum albumin >= 2 g/dL (within 7 days prior to
             enrollment)

          -  Activated partial thromboplastin time (aPTT) =< 1.5 x ULN (within 7 days prior to
             enrollment)

          -  International normalized ratio (INR) =< 1.5 (within 7 days prior to enrollment)

          -  Patients must be able to swallow intact tablets

          -  All patients and/or their parents or legally authorized representatives must sign a
             written informed consent; assent, when appropriate, will be obtained according to
             institutional guidelines

        Exclusion Criteria:

          -  Pregnant or breast-feeding women will not be entered on this study due to risks of
             fetal and teratogenic adverse events as seen in animal/human studies; pregnancy tests
             must be obtained in girls who are post-menarchal; women of child-bearing potential and
             their partners should agree to use two (2) highly effective forms of contraception
             throughout study participation and for at least one (1) month after the last dose of
             olaparib; male study participants should avoid fathering a child or donating sperm
             during the study and for three (3) months after the last dose of olaparib

          -  Concomitant medications

               -  Corticosteroids: patients receiving corticosteroids who have not been on a stable
                  or decreasing dose of corticosteroid for at least 7 days prior to enrollment are
                  not eligible; if used to modify immune adverse events related to prior therapy,
                  >= 14 days must have elapsed since last dose of corticosteroid

               -  Investigational drugs: patients who are currently receiving another
                  investigational drug are not eligible

               -  Anti-cancer agents: patients who are currently receiving other anti-cancer agents
                  are not eligible

               -  Anti-GVHD agents post-transplant: patients who are receiving cyclosporine,
                  tacrolimus or other agents to prevent graft-versus-host disease post bone marrow
                  transplant are not eligible for this trial

               -  CYP3A/CYP3A4 agents: patients who are currently receiving drugs that are strong
                  and moderate inducers or inhibitors of CYP3A or CYP3A4 are not eligible; strong
                  inducers or inhibitors of CYP3A4 should be avoided from 21 days prior to
                  enrollment to the end of the study

          -  Patients who have an uncontrolled infection are not eligible

          -  Patient who are known to be serologically positive for human immunodeficiency virus
             (HIV)

          -  Patients with known active hepatitis (i.e. hepatitis B or C)

          -  Patients who have received a prior solid organ transplantation are not eligible

          -  Patients with symptomatic uncontrolled brain metastases; a scan to confirm the absence
             of brain metastases is not required; the patient can receive a stable dose of
             corticosteroids before and during the study as long as these were started at least 4
             weeks prior to enrollment; patients with spinal cord compression unless considered to
             have received definitive treatment for this and evidence of clinically stable disease
             for 28 days

          -  Patients with known symptomatic Fanconi anemia (FA), ataxia-telangiectasia (A-T)
             syndrome, Bloom syndrome (BS) and Nijmegen breakage syndrome (NBS) are not eligible
             (asymptomatic carriers are acceptable)

          -  Major surgery must not have occurred within 2 weeks prior to enrollment and patients
             must have recovered from any effects of any major surgery

          -  Patients who in the opinion of the investigator may not be able to comply with the
             safety monitoring requirements of the study are not eligible