What sort of cooling system are you running? Stock air, this is going to generate a lot of heat. The GTX960 is a warm-running card out of the box, and overclocking it is going to warm it up even more.
Also, who is your card's manufacturer? There are a number of tools available to overclock these with, some easier to use than others, some with better options than others.
As a general rule, I don't normally overclock anything, but I've been building PC's for close to 30 years, so I've learned a few things.