Yeah, to be honest, it seems like a lot of STI transmissions happen unknowingly. Two people I know have disclosed to me they have STIs and in both cases, the sexual partner was "known" to be clean (e.g., testing was done, no symptoms).
For example, STIs like herpes (which the majority of people have orally) and HPV (which, AFAIK, is the most common STI) can be spread when symptoms aren't visible due to viral shedding. Those are both spread via skin-to-skin contact and thus can be spread during oral. And I think a lot of people have these STIs without realizing it, as you may never show symptoms for herpes after your initial infection (which may have happened as a child) or you get infected while your sexual partner isn't showing symptoms (so you don't expect it). As far as I know, neither of these STIs has good, reliable tests you can take. I also just read that 1/8 HIV-positive people don't know they have it. but that's probably because of the latency period others have mentioned.
Now, I'm really good at scaring people, and I don't mean to scare anybody out of having sex. Herpes isn't life-threatening and very few cases of HPV will actually lead to anything dangerous. Even HIV has medical treatments these days. I'm just saying make sure you're educated and know what you're doing before you do it. If you know the risks before making choices, then all the more power to you and I hope you enjoy it.
EDIT: Oops, this post is coming a few posts late. I suppose I'll get off my soapbox now.