Patients with moderate to severe COPD living below 800 m, will be recruited to participate in a randomized cross-over field trial evaluating the hypothesis that exercise capacity during a 2 day sojourn at moderate altitude is improved by nocturnal oxygen therapy via a nasal cannula. Outcomes will be assessed during 2 days in Zurich (490 m, low altitude baseline), and during 2 days at St. Moritz Salastrains (2048 m).