More than 280,000 surgeries are performed in New Zealand every year. The amount of oxygen given to patients during and after major operations may affect their risk of post-operative complications. Given the large number of operations being performed, establishing the optimal approach to oxygen therapy in patients undergoing major surgery is a public health priority. The Hospital Operating Theatre- Randomised OXygen trial (HOT-ROX) is a 1760-participant randomised controlled trial (RCT). It will establish whether careful titration of oxygen improves outcomes compared with using oxygen liberally for patients having major surgery. Before proceeding with HOT-ROX, this feasibility study is being conducted to evaluate a range of issues including protocol compliance, recruitment rate feasibility, protocol safety, and ease of protocol implementation.