Just to be clear. Are you manually OC'ing CPU or GPU(I'm not referring to the OC edition of the card). As drivinfast247 said, if there is a manual OC, then set everything to stock and try again. This applies to ram too.
If that doesn't help, i'd suggest you run something like HWMon/Info and monitor the temps/voltage/load on the GPU, and the rest of the system too. If the temps are specially high, it could cause artifact in the same way a poor manual OC can do.
Listing your specs in full will really help too.
Try the obvious stuff. Update GPU driver to the latest, update system drivers, and even do a bios update if there is a new one.
It could be memory, you can test that with memtest86+ and run a few passes at stock speed. It's unlikely a system memory issues would cause on screen artifacts. But it could cause random crashes.