How does low oxygen make you feel and how would you know?

Hmmm.Why do you ask?: Short of breath and unable to get up and move around. Usually lips turn blue and you develop altered mental status and chest tightness. It's typically not subtle. Ever seen a fish out of water? That's what people who are hypoxemic look like until they tire out and stop breathing. Then they appear peaceful for awhile before it's obvious they're dead. If you think someone is in trouble 911.Emergency

Additionally...: When hypoxia develops gradually, the symptoms include light-headedness / fatigue, numbness / tingling of extremities, nausea or even elation.