performing oral sex on someone who is currently infected is a realistic transmission; even if they aren't showing symptoms. You don't have to have symptoms to have an STD.
there are studies in which the STDs I've listed above can be transmitted... HIV is not listed because it isn't transmitted via oral sex.
Thanks for the quick answer! However my 2nd question was more geared towards the infection risk while performing oral sex like, if you perform any type of oral sex and your partner doesn't show any symptoms, is it still (in real life) possible to get infected or is the risk only theoretical? Are there actually any studies on the transmission probability of STDs through oral sex like they have been done for HIV?
well lets start with this was a low risk for any std transmition but a small risk for the following, Gonorrhea, NGU, Trick, HSV, and syphilis. at this point you can test for the first three, if you are highly concerned of the other two then you must wait 3 months for HSV blood test and 6-8weeks for syphilis blood test.
it is known that if no symptoms of sores occur with in the first 20-30 days that most do not worry about transmition. as for the gono, ngu and trick you could be infected with those for months without presenting any symptoms.
protected sex with your GF would prevent from any transmition of any STD.