Oral sex exposes you to gonorrhea, herpes and syphilis, but the general risk is really low.
You can test as follows:
-Herpes, 3 months after the encounter.
-Gonorrhea, 3 weeks after the exposure.
-Syphilis, most people test positive between 3 and 6 weeks after the encounter (in case there is infection) but some people take longer (until 3 months).
All the best.