Paying extra money for more vram is generally not worth it. For a gtx 770 it isn't. You'd only need 4gb for 4k gaming, and even then 3gb could be enough. But the problem is that a 770 is a little to slow for that kind of stuff. as for the "oc" names is just a marketing thing, telling that their cards are overclocked at higher frequencies compared to the reference gtx 770. Don't worry about that.
If you're gonna get a gtx 770 just get the cheapest one from a well known company like Sapphire, Asus, msi and so on. Pny are of course fine too.