Olaparib is available as a film-coated tablet containing 150 mg or 100 mg of olaparib. Subjects will be administered study treatment orally at a dose of 300 mg twice daily (bid). The planned dose of 300 mg bid will be made up of two x 150 mg tablets twice daily, with 100 mg tablets used to manage dose reductions

Enzalutamide:

Enzalutamide is available as capsules or tablets containing 40 mg of enzalutamide. Subjects will be administered study treatment orally at a dose of 160 mg once daily.

Abiraterone acetate with prednisone: Abiraterone acetate is available as tablets containing 250 mg or 500 mg of abiraterone acetate. Subjects will be administered study treatment orally at a dose of 1,000 mg once daily in combination with prednisone 5 mg administered twice daily orally. Prednisolone is permitted for use instead of prednisone if necessary.

300 mg (2x 150 mg tablets) twice daily

The time from randomisation until the date of objective radiological disease progression (determined by RECIST 1.1 (soft tissue) and Prostate Cancer Working Group 3 (PCWG-3) (bone)) or death (by any cause in the absence of progression) regardless of whether the patient withdrew from randomised therapy or received another anti-cancer therapy prior to progression. Progression is defined using (i) Response Evaluation Criteria In Solid Tumors Criteria (RECIST v1.1) for soft tissue, as a >=20% increase in the sum of diameters of target lesions and an absolute increase of >=5mm taking as reference the smallest sum of diameters since treatment started including the baseline sum of diameters; (ii) Prostate Cancer Working Group 3 (PGWG-3) for bone as >= 2 new bone lesions on the 1st week 8 scan compared to baseline. The confirmatory scan, >=6 weeks later, must show >=2 more new bone lesions (for a total of >=4 new bone lesions since baseline).

ORR is the percentage of patients with at least one visit response of Complete response (CR) or Partial response (PR), in their soft tissue disease assessed by Response Evaluation Criteria In Solid Tumors Criteria (RECIST v1.1), in the absence of progression on bone scan assessed by Prostate Cancer Working Group 3 (PCWG3)). Per RECIST v1.1, CR=Disappearance of all target lesions; PR = >=30% decrease in the sum of diameters of target lesions; For each treatment group, ORR is the number of patients with a CR and PR.