When does low blood oxygen levels result in a coma? (i know it is under 74 because i was still sort of awake then although i suspect not much under.)

Well this varies: This varies a bit from person to person, so there is not really a number. It sounds like you are talking about the pulse oximetry reading, right? This is not used to generate concern about a coma. The pulse ox reading varies and is somewhat inaccurate. For most people, having insufficient oxygen for over 5 minutes results in brain damage, which may cause a coma.