Oral sex is low risk - not zero, but low enough that you shouldn't loose sleep over it, especially if you're not showing any symptoms (discharge, burning, sores/lesions). If you're sexually active, you should be getting an STI screening as a normal part of your annual physical (gonorrhea, chlamydia, HIV, and syphilis). But, in this situation, you can safely assume that you're STI free based on your lack of symptoms.