this looks like a super cool concept. The big problem is I have yet to play OSE because I'll admit it I'm a little nervous about the level of complexity. To use this would I only need the free basic rules? Or would I need the advanced rules?
The free rules (from their online SRD) have everything you need. The "Advanced Fantasy" rules simply add new classes, race+class options, and more monsters and treasure. But there's already a lot in the "Basic" form of the game.