The OGL covers specific verbatim rules text, which is copyrighted even if the mechanics aren't. If you want to rewrite every rule in D&D, you can, but it's easi

The OGL covers specific verbatim rules text, which is copyrighted even if the mechanics aren't. If you want to rewrite every rule in D&D, you can, but it's easier to copy text or reference specific identifiable terms in D&D with the OGL. The OGL also defines what's protected identity, like the word "beholder" for the monster or specific book titles, and what's not, like the spell named "magic missile". Regardless of trademark or fair use, if you agree to the OGL then you also agree to not use declared protected identity terms. For D&D, that generally means things related to their setting or branding.