There is only one known case of a person being infected with HIV through oral sex. In that case (as far as I remember) she had a major gum disease where her gums were constantly bleeding! There is not enough of the HIV virus in saliva for it to infect a HIV- person. The only bodily fluids that could infect someone are blood, semen and vaginal fluids and breast milk. For you to be infected one of these fluids, containing the HIV virus, would have to come into contact with one of your own bodily fluids.
If you are engaging in oral sex, and your gums bleed or ye have cuts in your mouth, and you let a HIV+ bloke cum in your mouth, technically, it possible to become infected.