Oblivion uses DirectX.

If other OpenGL apps seem to work fine but Poser 7 consistently crashes, then it could be something to do with the app itself (it's clear that thing still has a few bugs that need to be fixed).

If you get the same problems even when you are running your video card at the stock speed, then it's not even the overclocking that's causing it, just Poser 7's bad programming.

Though it's probably the case that nVidia deliberately rates their 7600 parts at lower speeds than they're actually capable of so they don't compete with their higher-end 7900 parts.