@PresGas Yes. Part 4 gives you a licence to Use; Part 1 defines Use to include distribute, copy, edit, format, and modify.

That means for content designated OGC you can modify and distribute those modifications, alongside your own work. D&D 3rd Edition gave a massive boost to the RPG industry by releasing their core rules OGC and it spawned a whole host of D20 spin offs by other publishers.

Note that product identity, specific names, etc, are not included in OGC, so the OGC version of Mordernkainen's Magnificant Mansion was simply called "Magnificent Mansion", etc.

I think Part 2 also makes the OGL kind of copy-left, as any OGC that you Use, e.g. modify and distribute, must also have the OGL.

A product can have both product identity parts (usually the fluff) and OGC parts (the rules).