Inclusion Criteria:  Patients aged 21-85 years old presenting for elective surgery. Suspected/possible OSA patients who are enrolled in the "fast-track OSA protocol" in the pre-assessment clinic Patients started on CPAP therapy at least one day pre-operatively Patients who are admitted to the High Dependency unit for postoperative monitoring.  Exclusion Criteria:  Patients with nasal obstruction. Patients undergoing nasal or facial surgery, or surgery to treat OSA (eg. uvulopalatopharyngoplasty). Patients who already require a higher oxygen concentration (FiO2 > 0.21) preoperatively.