If either:
1/ Your game is heavily CPU-intensive and doesn't need much graphic power ( some MMORPG games, like TERA for example ).
2/ Your CPU is bottle-necking the GPU ( full load on 1/2/3/4 cores of the CPU ( depends on how many CPU cores that game you play can ultilize ) but only 80% or less GPU usage while gaming ).
Then yes, OCing the CPU will help improving the FPS.