Oral sex (both active and passive) exposes you to gonorrhea, syphilis and herpes, but the general risk is much lower than through penetrative (anal/vaginal) sex.
If you perform normally oral sex on people you don't know you should test periodically, at least once per year, for all those stds. If you stop those activities and you want to test to know if you are clear, you should test as follows:
-3 weeks after the oral encounter, test for gonorrhea.
-6 weeks after the encounter, for syphilis.
-For herpes, 3 months after the encounter.
Some people take longer to show a positive result for syphilis, so if yours is negative, it would be interesting to repeat the test 3 months after your last encounter.
All the best.