Exercising With Oxygen Therapy (EWOT) is becoming a popular form of oxygen therapy that uses oxygen from a tank or oxygen concentrator. A nasal canula or mask is used to get the oxygen into the nostrils. In either case, you are not breathing 100% oxygen. Flow rates vary from 3 to 8 lpm.
People with compromised breathing systems are on this form of oxygen 24/7 so I seriously doubt if you could overdo it to the point of poisoning, but frankly I don't know.