Oral sex (mouth-penis, mouth-vagina): The risk from oral sex is very minimal as the mouth is an inhospitable environment for HIV, for several reasons. Saliva contains enzymes that break down the virus; also, the skin of the mouth is sturdier than in the anus or vagina. There are, however, a few documented cases where it appears that HIV was transmitted orally. These cases are all attributed to ejaculation in the mouth (i.e., exposure to semen, not exposure to vaginal fluid or pre-seminal fluid).
Receiving oral sex is not risky because one is exposed only to saliva.