Here is an informative article:
The Great Fellatio Debate: How Safe Is Oral Sex? - The Body
We don't use condoms for oral. Is it possible to transmit HIV via oral sex? In extremely rare cases, yes. How much risk? Antidotally, Magic Johnson the basketball player was HIV positive for ~10 years, having unprotected sex of all types with his wife, and she never got it. (COULD you get HIV from a single sexual encounter? - yes.)