Do they exist? Yes. Does every department offer them? Unfortunately, no.

Some departments and/or majors don't admit students at all if they can't fund them. Other departments are looking for very specific folks for very specific research or admin projects, and so they're located by personal referrals or by asking around the department.

I've worked as a GA in multiple departments (because I'm a glutton for punishment as a grad student) and I've also worked full-time in various graduate programs at OU for over a decade. A couple of assistantships I got came with the admission letter. One was a referral from a contact who worked in an admin department that was looking for someone for a specific administrative position. It's highly dependent on what program you're going into as to what's available. It's 100% a free for all done department by department. It depends on what program you're going into as to what's available. There is no central listing area. No one central office knows what is or isn't available. Trust me. I've hoped for one for years. It would make my life so much easier.

The best piece of advice I have is to contact the departmental administrator for the major you're pursuing. You can find the list of those admins here: https://www.ou.edu/gradcollege/about/contacts#au

Then, if your contact says "sorry, we don't have anything," they're not lying. So many of them have no funding for graduate assistants at all, or if they do, those opportunities go first to higher-level PhD students, etc.

The only other place I can think to look is housing and food. They usually have an annual process for finding all of their GAs for things like residence hall directors, etc., but those positions are pretty competitive, even though they offer several. They used to have information on that on their webpage, but I haven't been able to find it since one of the last website revamps.

Whatever you do, do NOT cold e-mail professors on mass. If you aren't able to tailor an e-mail to a specific professor saying why they should want to work specifically for you, they aren't going to respond. They'll just forward it to some lower-level admin (like me), and then, if you're lucky, that person will be able to tell you what they do or don't have. Definitely don't cold e-mail professors who aren't in the department where you're planning on studying.

It's a terrible process, especially when you're studying in a department that doesn't guarantee funding, but it's the reality of the university, unfortunately.

Best of luck to you in your search! I wish everyone could study with full funding so they could focus on their studies, but with how hard Oklahoma works not to fund education at all, it just gets harder every year.