Oral sex exposes you to gonorrhea, syphilis and herpes, but the risk is really low.
You can test for gonorrhea 3 weeks after the encounter, for syphilis after 6 weeks (and if you get it negative, it would be interesting t re-test 3 months after the exposure, as there are people that take longer to show a positive result). For herpes, you can test 3 months after the encounter.
All the best.
Dragon.