An exhaust system for a vehicular compression ignition engine, preferably for a heavy-duty diesel vehicle, for treating an exhaust gas comprising inter alia oxides of nitrogen (NO x), which exhaust system comprising a composite zone-coated, dual-use ammonia and nitric oxide oxidation catalyst, a first injector for a nitrogenous reductant or a precursor thereof connected to a source of nitrogenous reductant or nitrogenous reductant precursor, which first injector being arranged to inject the nitrogenous reductant or nitrogenous reductant precursor into a flowing exhaust gas upstream from the composite oxidation catalyst and a first substrate comprising a selective catalytic reduction catalyst disposed between the first injector for a nitrogenous reductant or a precursor thereof and the composite oxidation catalyst, wherein the composite oxidation catalyst comprises: a substrate, optionally a honeycomb flow-through substrate monolith, having a total length L and a longitudinal axis and having a substrate surface extending axially between a first substrate end and a second substrate end; two catalyst washcoat zones wherein a first catalyst washcoat zone comprises a first catalyst washcoat layer comprising a refractory metal oxide support material and two or more platinum group metal components comprising both platinum and palladium supported thereon; and a second catalyst washcoat zone comprises a second catalyst washcoat layer different from the first catalyst washcoat layer, which second catalyst washcoat layer comprising a refractory metal oxide support material and one or more platinum group metal components supported thereon, which two catalyst washcoat zones being arranged axially in series on and along the substrate surface, wherein the first catalyst washcoat zone has a length L 1, wherein L 1 < L, is defined at one end by the first substrate end and at a second end by a first end of the second catalyst washcoat zone having a length L 2, wherein L 2 < L, and wherein the second catalyst washcoat zone is defined at a second end thereof by the second substrate end; and a washcoat overlayer extending axially from the first substrate end for up to 200% of the axial length of the underlying first catalyst washcoat layer, which washcoat overlayer comprising a particulate metal oxide loading of > 48.8 g/l (>0.8g/in 3), wherein the particulate metal oxide is an aluminosilicate zeolite including at least one of copper, iron and manganese, wherein a total platinum group metal loading in the first catalyst washcoat zone defined in grams of platinum group metal per litre of substrate volume (g/l) is greater than the total platinum group metal loading in the second catalyst washcoat zone, wherein the first substrate end of the composite oxidation catalyst is oriented to an upstream side. a substrate, optionally a honeycomb flow-through substrate monolith, having a total length L and a longitudinal axis and having a substrate surface extending axially between a first substrate end and a second substrate end; two catalyst washcoat zones wherein a first catalyst washcoat zone comprises a first catalyst washcoat layer comprising a refractory metal oxide support material and two or more platinum group metal components comprising both platinum and palladium supported thereon; and a second catalyst washcoat zone comprises a second catalyst washcoat layer different from the first catalyst washcoat layer, which second catalyst washcoat layer comprising a refractory metal oxide support material and one or more platinum group metal components supported thereon, which two catalyst washcoat zones being arranged axially in series on and along the substrate surface, wherein the first catalyst washcoat zone has a length L 1, wherein L 1 < L, is defined at one end by the first substrate end and at a second end by a first end of the second catalyst washcoat zone having a length L 2, wherein L 2 < L, and wherein the second catalyst washcoat zone is defined at a second end thereof by the second substrate end; and a washcoat overlayer extending axially from the first substrate end for up to 200% of the axial length of the underlying first catalyst washcoat layer, which washcoat overlayer comprising a particulate metal oxide loading of > 48.8 g/l (>0.8g/in 3), wherein the particulate metal oxide is an aluminosilicate zeolite including at least one of copper, iron and manganese, wherein a total platinum group metal loading in the first catalyst washcoat zone defined in grams of platinum group metal per litre of substrate volume (g/l) is greater than the total platinum group metal loading in the second catalyst washcoat zone, wherein the first substrate end of the composite oxidation catalyst is oriented to an upstream side.