how far can you go with OGL?


Hey All,
I am An old grognard and remember when books like OGL players and GM guide were the main stay. A local game shop had an 80% off sale so I picked up the OGL core books because I liked the OGL Ancients. what I was wondering is how far can you take the rules? Can you use any D20 supplements with little effort? I have not read the books full thru but would like to know before hand what i can do with them.
It's more a question of what would you want to include with each of the OGL books. Now granted I haven't read through all the OGL books myself but they should be just straightforward D&D style D20 so all the books should be compatible.
So if I pick up a D20 book like Warcraft or any of the books that say they need the d20 players hand book then the OGL should be fully compatible.
I've been struggling to adapt some third party stuff (Necromancer's Mesopotamia and Necropolis campaigns) to OGL Ancients, BUT it's hard work.

Spells are very different in operation, armour is completely different ( and combat is much more protracted, especially coverage) and there's not the same balance wrt the different classes. And there are no Prestige Classes, so I don't know how easy it would be to maintain balance if you took them from other sources, especailly on the magic using side.

Some designer notes on converting standard classes and monsters to OGL Ancients, which inspite of its flaws has masses of atmosphere!

By the way, ERRATA for OGL Ancients would be nice too!
What about the core books? Could pick up any of the source books out there and use OGL players hand book or OGL modern with them?