If you got identical FPS on battlefield 2142, this doesn't meant that you will get identical FPS with oblivion.
Battlefield 2142 has a huge map, so this means that the performance of BF2142 depends greatly on the CPU and the ram. However, the the lighting effects, texture size, shaders are horrible in BF2142 compared to oblivion. BF2142 doesn't have the ability to use 8800GTS to it's full potential. But oblivion will be able to use 8800GTS to it's full potential.