An air-cooled oil cooler for the wall of an air flow passage of a gas turbine engine is provided, the oil cooler allowing improved heat exchange performance to be obtained. This is achieved by: (I) diverting air flow from the inner side of the oil cooler towards the outer side of the oil cooler, (ii) enhancing heat exchange performance at the leading edge of the cooler, and/or (iii) suppressing airflow wake at the trailing edge of the cooler.