There's only one Open Gaming License. But yes, the idea is that you can use OGC from as many products as you like. Check out the lengthy list in some Pathfinder products - some of them have a a whole page full of products listed in their s15.
There are about half a dozen open licenses used in the gaming industry...
The Wizards OGL 1.0a (W-OGL henceforth) is actually one of the more restrictive.
Also used within the gaming industry:
- Creative Commons licenses. CC is a collection of related licenses, not a singular one. Most commonly used is CC-NC-BA-SA (non-commercial by attribution share alike). Has its own requirements for inclusion. CC is a collection of related licenses, not a singular one.
- GNU Free Document License - not used much, but several early games used it. Permits ONLY EXACT duplication of covered text, allows additions
- EABA Open Supplement License - Can't reprint the core, but can produce supplements based upon it.
- GNU Copyleft- which disclaims all rights except attribution of source. Few of those floating around, but it was used before the W-OGL
- Wizards GSL - which authorizes supplements but not copying the core text.
- Several other system specific open licenses such as...
- the custom license for BTRC's Epiphany (specifically, it was a non-commercial open supplement license)
- The Traveller Fair Use Provision (another variation on non-commercial open supplement, but allowing limited text copying)
- The Traveller Cloning Policy (now deprecated) that allowed copying whole materials published by GDW with the Traveller Trademark, but only one copy per original allowed to be given away. (was in effect during the timeframe 1998-2004)
And then the variety of supplemental licenses that restrict the above but allow use of compatibility marks. The (now deprecated) D20 STL, the Traveller Logo License, the Pathfinder Logo License, the Foreven Free License, the RuneQuest Logo License (now deprecated)...
The OGL by wizards is the only one that applies directly to D&D as we know it; be warned that not all open content is under the W-OGL...
Mixing can be problematic because the majority of them do not allow use without passing rights to that use onward, and several require that the entire work be placed under the same licensing...
If it's under the W-OGL, it's fair game in a W-OGL based product, unless it's defined as product identity.