Introduction Despite the high prevalence of obstructive sleep apnoea (OSA) in obese patients undergoing bariatric surgery, OSA is undiagnosed in the majority of patients and thus untreated. While untreated OSA is associated with an increased risk of preoperative and postoperative complications, no evidence-based guidelines on perioperative care for these patients are available. The aim of the POPCORN study (Post-Operative Pulse oximetry without OSA sCreening vs perioperative continuous positive airway pressure (CPAP) treatment following OSA scReeNing by polygraphy (PG)) is to evaluate which perioperative strategy is the most cost-effective for obese patients undergoing bariatric surgery without a history of OSA. Methods and analysis In this multicentre observational cohort study, data from 1380 patients who will undergo bariatric surgery will be collected. Patients will receive either postoperative care with pulse oximetry monitoring and supplemental oxygen during the first postoperative night, or care that includes preoperative PG and CPAP treatment in case of moderate or severe OSA. Local protocols for perioperative care in each participating hospital will determine into which cohort a patient is placed. The primary outcome is cost-effectiveness, which will be calculated by comparing all healthcare costs with the quality-adjusted life-years (QALYs, calculated using EQ-5D questionnaires). Secondary outcomes are mortality, complications within 30 days after surgery, readmissions, reoperations, length of stay, weight loss, generic quality of life (QOL), OSA-specific QOL, OSA symptoms and CPAP adherence. Patients will receive questionnaires before surgery and 1, 3, 6 and 12 months after surgery to report QALYs and other patient-reported outcomes. Ethics and dissemination Approval from the Medical Research Ethics Committees United was granted in accordance with the Dutch law for Medical Research Involving Human Subjects Act (WMO) (reference number W17.050). Results will be submitted for publication in peer-reviewed journals and presented at (inter)national conferences. Trial registration number NTR6991.