I was wondering if I finish OWB's can I go back, and explore the rest of the grounds? I know I can go back to the Sink, but can I go out into the crater?

Yes you can. Explore and even complete quest.

Great thanks, was worried I was getting to far in the main quest .

About half way through, I was getting low on ammo. Well, low on 308...So I just completed the MQ, to ensure I had enough. Came back later, fully equiped, and completed the sink. I still have about 20% of the map left, and a handful of quest. Maybe 4 left.