Oral sex has a very small risk but finally a risk, there are a lot if variables that increse or reduce the risk, and there are a lot of studies, some said 0% risk others said 8% risk, but the CDC (by the way very conservative) says1 in 20000, in receiving oral sex an 1 in 10000 in giving oral sex (to man), that can be reduced with condoms, and increased if there is visible blood in the mouth or by eyaculating in the mouth, and the other variables are the viral load of the hiv + partner (high at