I don't use WotC's text or tables, and I don't use their unique and characteristic IP, such as beholders, mind flayers, displacer beasts, or the Forgotten Realm. To the extent that I use the same mechanics as older editions of D&D, I use my own wording to describe them. As such, the OGL isn't necessary for what I do, in the same way that somebody else could come along and write a 100% clone of Stars Without Number just by using their own words and staying away from my specific setting and proper-noun IP.
I don't use the OGL unless it's strictly necessary, such as on my Labyrinth Lord products. It's a further layer of legal complication that I don't need.