If you need to you can heat up the oxygen to increase it's volume until the pressure is correct. I forget the reason that it's easier to do with methane than o

If you need to you can heat up the oxygen to increase it's volume until the pressure is correct. I forget the reason that it's easier to do with methane than oxygen but it's possible to do with oxygen too if you really need to. ULA's new ACES upper stage is planned to use autogenous pressurization for both it's oxygen and hydrogen tanks.