Once a god chooses an oracle, they can't stop the oracle from leveling, actually! If the oracle figures out how! They can just grab the power away from the god! Don't want to spare the power to make your oracle ninth-circle? Too bad! Once you've chosen an oracle, and the oracle has earned that much power, they get to take the power to become ninth-circle whether you like that or not! You'll just have to unchoose a lot of your other clerics!
That's where Erecura actually came from, by the way! Onnnneee time Pharasma tried choosing an oracle of Her own! Then by the time Erecura finished ripping Pharasma's power away from Her, Erecura had turned into a god Herself, so Pharasma cursed Her to stay in Hell, and Erecura managed to seduce Dispater and get a garden there for Herself etcetera etcetera.