Hitachi EC-89 oil cap pops off
I have a Hitachi EC-89 at 120 psi the oil cap pops off.
That will happen if you are getting air blowby the piston seal, and that air flows into the tank, and, typically, if the oil fill tube cap has a vent hole that has been plugged by debris.
Check your oil fill cap to ensure the vent is clear. If there is a foam insert, wash the insert with warm, soapy water, wait for it to fully dry, and reinsert it.
If the compressor is running and you have significant piston blowby, then it's likely time for a pump teardown and piston seal replacement / fix.