How much of a gap do I leave between the wall and the OSB I am putting down on the concrete for a subfloor?
I did the same thing in my last house, using DeltaFL and 4x8 sheets of OSB.

 

Spacing

I used spacers to leave a 1/4" gap around the outside, and then framed all interior walls on top of the subfloor. 

I had laminate installed on some of it, and carpet on the rest. I can attest it's a great way to do a floor as it has the feel of an upstairs floor. I lived in that house for 4 years without any issues whatsoever. 

Screws

I screwed it all down, and ended up using at least 24 tapcons per sheet. My floor was really quite level, but any minor variation meant when you walked on that spot you could hear the clicking of the plastic sheet, so for a couple weeks after I had it installed I was still finding 'clicking' spots and adding tapcons.

The downside: it's backbreaking work. I would definitely invest in a rotary hammer drill. I helped a friend do his the same way, and we had a rotary drill that time, and it could do 4 or 5 holes in the time a normal hammer drill can do one. Total install time was about 10 hours to do around 900 sqft (including everything) with 3 guys. I did mine mostly by myself working a couple hours every evening, about 350 sqft, and it took me a couple weeks.