En face or 3D volumetric OCT imaging during ophthalmic surgery may be performed with an OCT scanning controller that interfaces to an OCT scanner used with a surgical microscope. The OCT scanner may generate en face images before and after surgical operations, such as retinal membrane peeling, are performed. Using digital subtraction on the en face images, an overlay image indicative of the changes from the surgical operations to the eye may be generated and overlaid onto an optical image displayed to a user of the surgical microscope.