Detailed geological mapping incorporated with interpreted aeromagnetic data of the Igbara-Oke and Owena area of south-western Nigeria provides a new insight into the lithology and structures in the area. The interpreted, derivatives maps such as Total Horizontal Derivatives (THD), Second Vertical Derivative (SVD), Regional Magnetic Anomaly (RMA) were analyzed for lithology delineation. For structural delineation, 3D Euler deconvolution, Source Edge Detector (SED) and some derivative maps were used to highlight the various structures with their depths. Combination of information from field investigation and aeromagnetic data analyses revealed four main lithology contrary to a single lithology that is indicated in the regional geological map. Aeromagnetic data of the area has proved very useful in delineating geological contacts especially in inaccessible parts, such as the Ikeji Forest Reserve and places with limited outcrop exposures such as the swampy areas around Odunwo and Aba Sokoto.