The invention relates to new olaparib oxalic acid cocrystals. The olaparib oxalic acid cocrystals vary in ratio from 1:1 to 2:3 olaparib:oxalic acid. In particular, the invention relates to a 2:3 olaparib oxalic acid cocrystal, a 1:1 olaparib oxalic acid cocrystal, and a 2:1 olaparib oxalic acid cocrystal. The invention also relates to pharmaceutical compositions containing an olaparib oxalic acid cocrystal of the invention and a pharmaceutically acceptable carrier. The olaparib oxalic acid cocrystals of the invention Got may be useful for the treatment of diseases that benefit from inhibition of poly (ADP-ribose) polymerase (PARP). These include cancer, fibrosis, inflammatory conditions (e.g. asthma, colitis, arthritis), neurological diseases (e.g. neurodegeneration, neurotrauma, stroke), cardiovascular conditions, ophthalmic degenerative diseases, vascular diseases (e.g. diabetic complications, atherosclerosis), and various forms of critical illness (e.g. septic shock, ALI, acute liver failure).