Well, that was fun
You can pretty much use the full repertoire of D&D (less setting IP) with the OGL. It’s not like you ever need to reproduce the text of the core rules; you just need to use the terminology, which is all available from previous SRDs. That’s how OGL publishers operate.If you don't want to use setting specific items (like places, NPCs, gods, etc. - this includes some monsters too) of any D&D setting, then go OGL; If you want to use any setting specific items go DMsGuild; if you want to use the full repertoire of D&D, but don't want to use setting specific items, go DMsGuild w/ generic setting.
@Retreater, your first step is to read the OGL and make sure you understand it. It’s pretty short and clear, and most questions you will have will be answered by that document.