I am almost done with OWB and I just found out about this perk.If I get it now does it still work in the main game?

The perk description says "All living creatures killed by the Courier" its not limited to enemies in the Big Empty. So yeah it should work fine in the main game.

It definitely works in the main game, I have characters who make their living harvesting and crafting the drops for sale.

I'll just post this here instead of making a new thread.... Does the "devour insects" part of Tribal Wisdom perk really work? I tried it on radroaches in Dead Money and my health bar didn't move at all. If I'm going to act and sound like a ghoul, I want something for my trouble. *shudder*

Yes, it definitely works. I'd have to look in the GECK, but I don't recall it giving that much of a health boost, which may be why it didn't appear to work in DM since if you're playing in hardcoe mode you take damage just for being there.

Yes, it works everywhere. Consider it the "no bad karma cannibal".