Yes, there is some risk for herpes, syphilis, oral gonorrhea and chlamydia for the one who is giving unprotected oral.
The risk is low, much lower than during vaginal or anal sex, but it is real.
There are documented case reports and real stories on Medhelp to back it up. How low the risk is depends on many things.
I assume you are asking not out of sheer curiosity. If you describe your exposure, our community members will help you to assess your risk.