There is a risk of transmission during unprotected oral sex. The San Fransisco City Clinic has put together a really nice STI (aka STD) Risk Chart and it's online:
This site breaks down different sexual acts, along the lines of biological, sexual organs, into categories of STI known risks, possible risks, and unknown risks.
It also notes that:
- Kissing, mutual masturbation, and frottage or dry humping are considered safer sex activities, with little to no risk of STD transmission.
- Using latex condoms (male or female) significantly reduces the risk of contracting STDs during anal, vaginal and oral sex.
- Washing hands and the genital area thoroughly before and after oral-anal sex reduces the risk of transmission of most of the listed STDs and conditions. Condom use reduces transmission risk even further.
This was a really wonderful question that continues to have a rather vague answer! HIV can definitely be passed on to either the person performing or receiving oral sex; however, it is only speculated that the risk of transmission is less than through unprotected anal or vaginal intercourse.
Thanks for asking,