Inclusion Criteria:

          -  Ability to understand and the willingness to sign a written informed consent document
             that is approved by the local institutional review board and HIPAA authorization for
             the release of personal health information.

          -  Histological or cytological proof of prostate adenocarcinoma (Note: small-cell
             carcinoma of the prostate is not permitted)

          -  Documented progressive mCRPC based on at least one of the following criteria:

               1. PSA progression defined as 25% increase over baseline value with an increase in
                  the absolute value of at least 2.0 ng/mL that is confirmed by another PSA level
                  with a minimum of a 1 week interval and a minimum PSA of 2.0 ng/mL.

               2. Progression of bidimensionally measurable soft tissue or nodal metastasis
                  assessed within one month prior to registration by a CT scan or MRI.

               3. Progression of bone disease (evaluable disease) (new bone lesion(s)) by bone
                  scan.

          -  Agree to undergo a biopsy of at least one metastatic site (fresh biopsy of primary
             prostate only allowed if there is clear local disease and no other measurable disease
             site or biopsiable bone lesion.) to determine DNA repair defects. (Please refer to the
             Laboratory Manual for specific procedures). However:

               1. Adequate archival metastatic or primary disease tumor tissue can be used if
                  available in lieu of a new biopsy. These patients will only be eligible for
                  protocol therapy if the biopsy has tumor that is positive for DNA repair defects.

               2. Patients with known DNA damage repair defects based on prior appropriately
                  validated metastatic or prostate tissue analysis may be used in lieu of new
                  biopsy/analysis based on central site evaluation of quality of the biopsy and
                  analysis.

               3. Patients with known germline DNA repair defects are eligible without a biopsy.
                  However it will be highly desirable that they undergo a metastatic (or fresh
                  prostate biopsy if there is clear local disease and no other measurable disease
                  site or biopsiable bone lesion) disease biopsy to better define the scope of the
                  DNA repair defects in the current disease context.

          -  ECOG status of 0-2 (Appendix A: Performance Status Criteria).

          -  Adequate organ function as defined below obtained within 14 days of registration:

        ANC > or = 1500/µl Hemoglobin ≥ 10.0 g/dL WBC > 3x10^9/L Platelet count 100,000/µl
        Creatinine ≥51 mL/min estimated using the Cockcroft-Gault equation Potassium ≥ 3.5 mmol/L
        (within institutional normal range) Bilirubin within normal institutional limits (or <2X
        the upper limit of normal (ULN) in those with Gilbert's disease) AST (SGOT) / ALT (SGPT) ≤
        1.5x institutional ULN unless liver metastases are present in which case it must be ≤ 5x
        ULN

          -  The effects of abiraterone, olaparib or the combination of both on the developing
             human fetus at the recommended therapeutic dose are unknown. Men must agree to use
             adequate contraception prior to study entry, for the duration of study participation
             and for at least 3 months thereafter.

          -  Patients must discontinue antiandrogen therapy (i.e., flutamide, bicalutamide,
             nilutamide) for at least 4 weeks prior to registration with no evidence of a falling
             PSA after washout.

          -  Serum testosterone < 50 ng/dL. Patients must continue primary ADT with an LHRH
             analogue (agonist or antagonist) if they have not undergone orchiectomy.

          -  Able to take oral medication without crushing, dissolving or chewing tablets.

          -  Patients must have a life expectancy ≥ 6 months.

          -  Patients may have received prior radiation therapy or surgery. However, at least 14
             days must have elapsed since completion of radiation therapy or surgery and patient
             must have recovered from all side effects at the time of registration (e.g. back to
             baseline or grade 1) .

          -  Patient is willing and able to comply with the protocol for the duration of the study
             including undergoing treatment and scheduled visits and examinations including follow
             up.

        Exclusion Criteria:

          -  Prior exposure to CYP17 (other than ketoconazole) or PARP inhibitors for prostate
             cancer. Patients with prior exposure to ketoconazole are eligible.

          -  Prior chemotherapy for castration resistant disease. Chemotherapy given in the
             hormone-sensitive setting is permissible if stopped at least 4 weeks prior to
             registration.

        Note: Patients can receive a stable dose of bisphosphonates for bone metastases, including
        zoledronic acid, or denosumab before and during the study as deemed appropriate by the
        treating physician.

          -  Prior exposure to enzalutamide, ARN-509 or other investigational AR-directed therapy
             in the setting of mCRPC.

          -  Patients with a currently active second malignancy excluding non-melanomatous skin
             cancer or superficial transitional cell carcinoma.

        Note: Patients are not considered to have a "currently active" malignancy if they have
        completed all therapy and are now considered without evidence of disease for 1 year.

          -  Patients receiving any other investigational agents. Any prior investigational agents
             must be stopped at least 14 days (2 week washout) prior to registration.

          -  Patients who have received itraconazole, ketoconazole, or fluconazole within 3 weeks
             prior to registration or those who have not recovered (i.e., back to baseline or Grade
             1) from AEs due to agents administered more than 3 weeks earlier.

          -  Patients with a history of active seizures (or a single confirmed seizure event) in
             the last 2 years from the time of registration.

          -  Patients with a history of pituitary or adrenal dysfunction or active or symptomatic
             viral hepatitis or chronic liver disease are not eligible.

          -  Patients with active brain metastases. A scan to confirm the absence of brain
             metastases is not required for asymptomatic patients.

          -  History of allergic reactions attributed to compounds of similar chemical or biologic
             composition to olaparib or abiraterone.

          -  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 (untreated or unstable
             within at least 28 days prior to registration), superior vena cava syndrome, and
             extensive bilateral lung disease on HRCT scan or any psychiatric disorder that
             prohibits obtaining informed consent.

          -  Patients with prolonged pre-existing hematological toxicities including known
             indicators of bone marrow failure or abnormality.

          -  Patients with myelodysplastic syndrome / acute myeloid leukemia.

          -  Patients may continue on a daily Multi-Vitamin, calcium and Vitamin D, but all other
             herbal, alternative and food supplements (i.e. PC-Spes, Saw Palmetto, St John's Wort,
             etc.) must be discontinued before starting protocol treatment. Hormonal-acting agents
             such as DES are forbidden during the trial and must be stopped prior to starting
             protocol treatment. No washout period will be required. Patients on megesterol acetate
             for hot flashes are allowed to continue therapy.

          -  Patients must stop taking ritonavir, idinavir, saquinavir, telithromycin,
             clarithromycin and nelfinavir 1 week prior to registration. Note: topical ketoconazole
             is permitted.

          -  Patients must stop taking phenytoin, rifampicin, rifapentine, rifabutin,
             carbamazepine, nevirapine, modafinil and St John's Wort (Hypericum perforatum) 3 weeks
             prior to registration.

        Patients must stop taking phenobarbitone 5 weeks prior to registration.

        Patients must stop taking all strong CYP3A4 inhibitors, including clarithromycin,
        telithromycin, nefazodone, itraconazole, ketoconazole, atazanavir, darunavir, indinavir,
        lopinavir, nelfinavir, ritonavir, saquinavir, and tipranavir, prior to registration.

          -  Patients must not be planning to receive any concurrent cytotoxic chemotherapy,
             surgery or radiation therapy during protocol treatment.

          -  Use of any prohibited concomitant medications within 7 days of registration.

          -  Patients who are HIV-positive on combination antiretroviral therapy because of the
             potential for pharmacokinetic interactions with olaparib. In addition these patients
             are at increased risk of lethal infections when treated with marrow suppressive
             therapy.

          -  Patients with known active Hepatitis B or Hepatitis C.

          -  Patients with baseline moderate to severe hepatic impairment (Child-Pugh Class B and
             C).

          -  Persistent toxicities (≥CTCAE Grade 2), with the exception of alopecia, caused by
             previous cancer therapy.

          -  Resting ECG with QTc > 470msec on 2 or more time points within a 24 hour period or of
             long QT syndrome.

          -  Patients with significant cardiac history including:

               -  Severe or unstable angina pectoris

               -  Uncontrolled hypertension (defined as systolic BP ≥ 160 mmHg or diastolic BP ≥ 95
                  mmHg). Note - Patients with a history of hypertension are allowed provided blood
                  pressure is controlled by anti-hypertensive treatment

               -  Atrial fibrillation or other cardiac arrhythmia requiring therapy.

               -  Heart disease as evidenced by myocardial infarction, or aterial thrombotic events
                  in the past 6 months

               -  Class II-IV heart failure (as defined by New York Heart Association) or a cardiac
                  ejection fraction measurement of less than 50% at baseline

          -  Blood transfusion within 30 days of consent.

          -  Previous allogeneic bone marrow transplant.

          -  Major surgery within 14 days of registration and patients must have recovered from any
             effects of any major surgery.

          -  Patients with any condition likely to interfere with absorption of the study
             medication.

          -  No other condition which, in the opinion of the Investigator, would preclude
             participation in this trial.

          -  Patients who have noncanonical DNA repair defects and extensive visceral disease or
             symptomatic bone disease requiring urgent tumor response.