Oral sex exposes you to syphilis, gonorrhea and herpes. The risk is low compared to penetrative sex, but it's not zero, You should test as follows:
-Gonorrhea, 3 weeks after the exposure.
-Herpes, 3 months after the exposure.
-Syphilis, 6 weeks after the exposure. If the result is negative, repeat the test 3 months after the exposure, as there are people who take longer to show a positive result.
All the best.