An oxidation catalyst for treating an exhaust gas from a compression ignition engine, which oxidation catalyst comprises: a first washcoat region comprising platinum (Pt) and a first support material, wherein the first washcoat region is substantially free of a hydrocarbon adsorbent; a second washcoat region comprising platinum (Pt), palladium (Pd) and a second support material, wherein the second washcoat region has a ratio by mass of platinum (Pt) to palladium (Pd) of 4:1 to 1:1; a hydrocarbon adsorbent; and a substrate; wherein the first washcoat region is arranged to contact inlet exhaust gas before the hydrocarbon adsorbent, and the first washcoat region is arranged to contact inlet exhaust gas before the second washcoat region.