O2 does indeed supplement performance at altitude. Seriously, how could it not?? If a racer uses it (properly) to boost performance prior to a race, they'll have some degree of an edge over the other racers who follow the rules and don't huff O2. O2 supplementation can also be used to compensate for not training enough at higher altitudes.
If the rules are changed to allow pre-race O2 supplementation... well, I'm not sure folks here would like the results. Because all the racers would soon be marching around the staging/start area with O2 tanks & masks, pre-race. Also, if O2 is allowed, you'd probably see some other changes to the sport - like changes to training regimes. If I were a racer, I'd like that because cardio work is so danged boring.
Then there's the issue of parity. For example, which world cup record is more significant: one set before O2 was allowed or one set after? I know what my answer would be to this...