OEM only means it's the card straight from Nvidia or AMD, nothing tweaked or changed by the distributor (no custom cooling/backplates, factory OC, etc.).
But a 660 OEM shouldn't be significantly worse than a 7870, no. Pretty much it's up and down. 7870 outperforms 660 in some games, 660 outperforms in other. I would recommend going for a custom one though, instead of using a reference cooler. Getting something with a custom cooler can really help keep temps down, anywhere from 10-20C. And even sometimes a nice factory overclock.