Augmented Reality (AR) in microscopic surgery has been subject of several studies in the past two decades. Nevertheless, AR has not found its way into everyday microsurgical workflows. The introduction of new surgical microscopes equipped with Optical Coherence Tomography (OCT) enables the surgeons to perform multimodal (optical and OCT) imaging in the operating room. Taking full advantage of such elaborate source of information requires sophisticated intraoperative image fusion, information extraction, guidance and visualization methods. Medical AR is a unique approach to facilitate utilization of multimodal medical imaging devices. Here we propose a novel medical AR solution to the long-known problem of determining the distance between the surgical instrument tip and the underlying tissue in ophthalmic surgery to further pave the way of AR into the surgical theater. Our method brings augmented reality to OCT for the first time by augmenting the surgeon's view of the OCT images with an estimated instrument cross-section shape and distance to the retinal surface using only information from the shadow of the instrument in intraoperative OCT images. We demonstrate the applicability of our method in retinal surgery using a phantom eye and evaluate the accuracy of the augmented information using a micromanipulator.