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 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 in addition to abiraterone.