RE: OC'ing the GPU(s) as long as the temps are high the risk to damaging a GPU is very low. It's no different than OC'ing the CPU. A Cheap and/or Undersized PSU has the greatest risk to damage a GPU. If the temps and/or OC is too great from the GPU(s) to handle then like a CPU it will 'drop-out' (often a RSOD) or 'throttle-down' to protect itself.
99% of the builds I do offer OC'ing profiles for both the CPU and GPU(s) so I'me very comfortable at least with the rigs I build OC'ing. Most are indeed fully blocked aka water cooled.
I recommend OC'ing non-reference GPU's and/or carefully monitoring the temps. MSI Afterburner has a 'graphical chart' so either run a GPU bench or Game for a while and then examine the temps carefully.
Reference GPU ; no so great for OC'ing
Non-Reference GPU ; great for OC'ing