I have no Idea what an O2 sensor really does, no ones ever told me, sorry i'm stupid So, what exactly does it do, and can I used my stock one on a homemade dp for a turbo set up i'm trying to finish piecing together? If not, where do I get one?

 

Just find a downpipe with a o2 sensor hole in it, BMC racing makes them too. They are in the classifieds,

 

Then do I just put my stock O2 sensor in that hole. If it's that easy, and I might be making my own dp, then I'll just drill my own hole.

 

It isn't JUST a hole. :P
You need an o2 bung that the sensor threads into. You can probably get one welded in at an exhaust shop for about $15.

 

Oooo, okay, thanks, I'll check it out.