Jesus, dude. We just had that long thread on why you should never do this much and you were quite active in it. You didn't do permanent damage, but I think you should stop anyway because you obviously lack the self control necessary to use responsibly.
This is not an issue with moderate use.
Personally, I'd stop using Nitrous Oxide regularly due not due to lack of Oxygen - but rather entirely due to it's possibility to cause Psychosis, muscle degradation, etc.
This is not an issue with moderate use.