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 a 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. Results: 44 subjects were analyzed. The mean age, women and BMI was 57.36 ± 13.8 years, 59.1 %, 38.38 ± 6.31 kg/m2 , respectively. With supplemental oxygen at 28 % and 50 %, there was a -0.011 decrease in arterial pH (P = 0.003), an increase of 1 mmHg in PaCO2 (P = 0.039), 16.6 mmHg in PaO2 (P = 0.001) 0.007 mmHg at the HCO3 (P = 0.795) and SO2 levels of 1.62 % (P = 0.029). Conclusion: Supplementary oxygen at 28 and 50 % in obese patients with PaCO2 greatest that 35 mmHg on heights over 2500 meters worsen hypercapnia on 1.00 mmHg.