I know this is a stupid question but can someone let me know a good reference with code I can take a look at that has Open-GL as there graphics API to make doom style 3-d maps. Maybe GitHub would have a reference I've learned Open-GL is easier to code with to make doom style maps. or any other website that has a source and a good reference to learn from would be awesome as well.

By Doom (based on your mention of 'Ultimate Doom') I assume you're referring to the earlier games and not the more recent ones (which are much more complex).

To help with searching, it's 'OpenGL' rather than 'Open-GL'. For clarity, you don't really use OpenGL to 'make' maps - generally you'd use a map editor or other content creation tool for that. But if you mean rendering, then OpenGL could be a candidate, depending on what platforms you're interested in targeting. (As for OpenGL being easier to code with, it depends on what you're comparing it to.)

The rendering requirements for something like the original Doom should be fairly modest. You might be able to find an existing code base to work from, but another option would be just to work on learning OpenGL itself. If your goal is more to get results than to learn low-level technology, you might prefer something like Unity or Unreal.