An exhaust system for a compression ignition engine comprising an oxidation catalyst for treating carbon monoxide and hydrocarbons (HCs) in exhaust gas from the compression ignition engine, wherein the oxidation catalyst is upstream of a selective catalytic reduction catalyst or a selective catalytic reduction filter (SCRF) catalyst and wherein the exhaust system further comprises an injector for injecting a nitrogenous reductant into exhaust gas downstream of the oxidation catalyst and upstream of the selective catalytic reduction catalyst or the selective catalytic reduction filter (SCRF) catalyst, wherein the oxidation catalyst comprises a first zone and a second zone having different compositions and in a side-by-side arrangement on a flow-through monolith substrate, wherein the first zone is upstream of the second zone; wherein the first zone comprises: a combination of a platinum (Pt) component and a palladium (Pd) component in a ratio by mass of the platinum component to the palladium component of 4:1 to 1:1 as the only platinum group metal component, an alkaline earth metal component comprising strontium (Sr) or barium (Ba) having a total amount of alkaline earth metal of 0.35 to 17.66g/L (10 to 500gft -3) and a support material comprising alumina doped with silica in an amount of 0.5 to 45 % by weight; wherein the second zone comprises a platinum (Pt) component as the only platinum group metal component; and wherein the platinum group metal components, the alkaline earth metal component and the support material are disposed on the substrate. wherein the oxidation catalyst is upstream of a selective catalytic reduction catalyst or a selective catalytic reduction filter (SCRF) catalyst and wherein the exhaust system further comprises an injector for injecting a nitrogenous reductant into exhaust gas downstream of the oxidation catalyst and upstream of the selective catalytic reduction catalyst or the selective catalytic reduction filter (SCRF) catalyst, wherein the oxidation catalyst comprises a first zone and a second zone having different compositions and in a side-by-side arrangement on a flow-through monolith substrate, wherein the first zone is upstream of the second zone; wherein the first zone comprises: a combination of a platinum (Pt) component and a palladium (Pd) component in a ratio by mass of the platinum component to the palladium component of 4:1 to 1:1 as the only platinum group metal component, an alkaline earth metal component comprising strontium (Sr) or barium (Ba) having a total amount of alkaline earth metal of 0.35 to 17.66g/L (10 to 500gft -3) and a support material comprising alumina doped with silica in an amount of 0.5 to 45 % by weight; a combination of a platinum (Pt) component and a palladium (Pd) component in a ratio by mass of the platinum component to the palladium component of 4:1 to 1:1 as the only platinum group metal component, an alkaline earth metal component comprising strontium (Sr) or barium (Ba) having a total amount of alkaline earth metal of 0.35 to 17.66g/L (10 to 500gft -3) and a support material comprising alumina doped with silica in an amount of 0.5 to 45 % by weight; wherein the second zone comprises a platinum (Pt) component as the only platinum group metal component; and wherein the platinum group metal components, the alkaline earth metal component and the support material are disposed on the substrate.