Oral sex exposes you to gonorrhea, syphilis and herpes, but the risk is really low. You can test as follows :
-3 weeks after the encounter, for gonorrhea.
-6 weeks after the encounter, for syphilis, and if your result is negative, you should repeat it 3 months after the exposure, as there are people that take longer to show a positive result.
-3 months after the encounter, for herpes.