According to Osho, when you practice anapanasati—simply watching the in-and-out breath—attention itself speeds up the breathing slightly, like a catalyst. This naturally raises oxygen intake, yet increasing oxygen is not the aim; witnessing is. With sustained, unbroken observation, breath becomes the seen, revealing your separation from bodily processes and awakening the silent, detached watcher.
Watching your breath makes it go a bit faster (so you get more oxygen), but the real purpose is to discover you are the watcher, not the breath.