Introduction: Supplementary oxygen on a high flow in obese patients with elevated PaCO2, carries the risk of worsening hypercapnia in patients at sea level. Nevertheless, at an altitude over 2 500 meters over sea level, is unknown the response to supplementary oxygen. Method: Randomized crossover clinical trial in subjects with BMI ≥ 30 kg/m2 and initial arterial blood gases with a PCO2 over 35 mmHg without supplementary oxygen, currently living for over a month at a height over 2 500 meters over sea level. Two tests were performed with supplementary oxygen of 28 % and 50 % to evaluate the PaCO2 on arterial blood gases.