When Wizards released the d20 SRD under the OGL there were several lines drawn that you don't cross. First not everything D&D is under the OGL which why you use the d20 SRD, any rules you derived also had to be open, and finally you don't use the D&D trademark or any other trademark unless you adhere to a second license.
Sure things like defining what open and what product identity, along with correctly setting up your section 15 (where you list all the OGL product you used in your product including your own) can leave your head scratching. But help is often just a post or email away to clarify this stuff.
The big problem is section 7 of the OGL.
7. Use of Product Identity: You agree not to Use any Product Identity, including as an indication as to compatibility, except as expressly licensed in another, independent Agreement with the owner of each element of that Product Identity. You agree not to indicate compatibility or co-adaptability with any Trademark or Registered Trademark in conjunction with a work containing Open Game Content except as expressly licensed in another, independent Agreement with the owner of such Trademark or Registered Trademark. The use of any Product Identity in Open Game Content does not constitute a challenge to the ownership of that Product Identity. The owner of any Product Identity used in Open Game Content shall retain all rights, title and interest in and to that Product Identity.(sigh)
Nostalgia marketing is fine ... to a point. And Die Cat's module is way over that point. Which is a real shame because the cover art is nice, the production values are good, and from all reports the module is a solid adventure. Now we wait with baited breath for Wizard's reaction or lack of it. Grognardia post articulates my fears quite well. I don't relish having to explain why what I write is not a rip off of Wizard's IP.
We see how this plays out.