This study aims to determine whether systemic oxygen therapy delivered through face mask would have beneficial effects on the healing of corneal epithelial wound in post-vitrectomy diabetic patients. It will be a prospective, randomised interventional clinical study conducted on diabetic patients indicated for vitrectomy. Rationale of this study is to see if systemic oxygen delivered via face mask will hasten the resolution of corneal epithelial defects, which may either be iatrogenic (surgically-induced intra-operatively to enable clearer visualisation for the surgeon) or spontaneous (due to corneal epithelial fragility which is commoner in diabetics)