The main thing one has to consider is that things can go two different directions.
The person giving oral sex risks the following"
Herpes,
Wart Viruses
Hepatitis in most varieties
Thrush
and surprisingly HIV is way down the list.
The transmissability will hinge on the condition of the inside of the mouth doing the receiving. Gingivitis and other gum conditions can increase risks of transmission radically. If the mouth is in perfect shape the risks of HIV are relatively low, but there are other things that are far worse.
The Person receiving oral sex risks the following:
NGU,
Gonorreah,
Hepatitis,
Herpes,
and a few others.
The risk of HIV transmission in this manner is extremely low.
One caveat: If at the time the sex takes place the person on the receiving end were to have even a mild case of urethritis, there are major problems in increasing the odds of transmission because with the urethra irritated blood transfer is possible.
It is all a risk no matter what you do. You have to always know the other person and you have to know your own body to minimize unprotected risk. Now, that being said, use of a condom is not all that hard and it can with regards to oral sex eliminate almost any chance no matter what. Perfect no, but in oral sex damn close.
Just use common sense..........