To a certain extent, everything from OD&D through 3.5e (and quite possibly 5e) are somewhat compatible. Back in my early gaming I was using D&D adventures in my AD&D game without batting an eye.
These days, I've used LL, C&C and S&W adventures and supplements in my ACKS campaign. Most of the conversion work I can do on the fly, but there are times I wish I had spent a few minutes ahead of time making sure things were ready for ACKS.
I've also grabbed some stuff from 3x and Pathfinder with minimal effort in converting, but I suspect the reverse would be more time consuming if not downright frustrating.
So, do you mix and match OSR/OGL rules in your games? Convert on the fly or scratch it out ahead of time?
Using Gary Gygax's D3 Vault of the Drow & Queen Of The Demonweb Pits With Astonishing Swordsmen & Sorcerers of Hyperborea - A Mini Campaign - "As a member of a bold party of adventurers, you and your associates have trekked far into what seems to be a whole underworld of subterranean tunnels -- a...
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