Source: http://www.poz.com/pdfs/P04-14p53.risk_transmission.pdf
Here's a great infographic that sums up the risks of HIV transmission from various sexual activities.
Here's another useful tool from the CDC that allows you to calculate your risks of getting HIV, assuming your partner is HIV positive: https://wwwn.cdc.gov/hivrisk/estimator.html
The CDC conducted a systematic review in 2014 to estimate the per act risk of getting HIV through various sexual activities, including oral sex.
They concluded that the risk of getting HIV from oral sex is low, citing a 10-year study of heterosexual couples with opposite HIV statuses where no new infections occurred from oral sex.
The risk may be higher or lower due to the following factors:
1. Whether you are receiving or giving oral sex
Receiving oral sex (having someone’s mouth on your penis or vagina): 0 or close to 0 risk. Saliva is not linked to HIV transmission. There have not been any cases of HIV transmission through receiving condomless oral sex.
Giving oral sex: Increased risk to the partner giving oral sex. There have been several documented cases of HIV infection in people giving oral sex.
2. Whether you are giving oral sex to a man or a woman?
Giving oral sex to a woman: 0 or close to 0 risk.
Giving oral sex to a man: Increased risk. Semen gets into your mouth more easily than vaginal secretions.
3. Whether semen or pre-ejaculate gets in your mouth?
If there is no semen/pre-ejaculate: 0 or close to 0 risk.
4. Oral hygiene of the person giving you oral sex?
Cuts, sores, or bleeding gums can all increase the risk of HIV transmission.
For further reading, this resource is a great reference for the discussion on the specific risks of HIV transmission during oral sex: http://hivinsite.ucsf.edu/InSite?page=pr-rr-05
