Allow me to provide a short example. Athletes who take oxygen from the sidelines ostensibly to "recover." A higher intake of oxygen will actually saturate the hemoglobin in the red blood cells and hinder exchange of carbon dioxide transfer from peripheral tissues. Sure, breathing pure oxygen reduces respiration rate - less huffing and puffing after scoring that touchdown - but has little value in allowing the muscles to recover.
Thus, as has been so often said - everything in moderation.