AMD Overdrive is meant for OCing your CPU.
Download MSI Afterburner and tweak ONLY your GPU core, raise it little by little and run Unigine Heaven benchmark. See if you gain any FPS or see any artifacts to determine if the OC is stable or not.
I can't stress enough ONLY tweaking your GPU core. If you increase the Voltage (if available) or memory frequency, you don't know what's causing instability.