This is a 2-part study in patients with metastatic CRPC. Part A is an open-label safety run-in study to assess the safety, tolerability and pharmacokinetics (PK) of olaparib when given in addition to abiraterone 1000 mg once daily. Part B is a randomised, double-blind, placebo controlled comparison of the efficacy, safety and tolerability of the dose of olaparib selected from Part A when given in addition to abiraterone, versus placebo given in addition to abiraterone. Abiraterone is indicated in combination with prednisone or prednisolone for the treatment of patients with metastatic CRPC. Prednisone or prednisolone 5 mg twice daily (bid) will be administered with the abiraterone in this study.