Air is not pure O2, or our tires wouldn't be happy for other reasons. Air is about 80% nitrogen.

Race teams use nitrogen for their tires because they also use it for their air tools; they can't run compressors in the pits.

Nitrogen is dirt-cheap. $40 to fill the tires with nitrogen is a huge rip-off. That's probably $.50 worth of the gas in there, and no extra work.

There will be no noticeable difference. The drier gas is slightly better, but even racers don't notice it. The difference you feel is probably over-inflated tires.