It's classified as a low-risk activity, but like crescendo said, you have to pay attention to how the virus is transmitted, not just to whether other people think the act is high risk.
You need three things to contract HIV: a partner with HIV, a fluid from that partner, and a way for the virus to get into your body. If you don't have any one of those things you absolutely cannot contract HIV. For instance, mutual masturbation with an HIV positive person will not facilitate the transfer of the virus unless your partner cums right into some open wound of yours, or something like that.
As for oral sex, as crescendo mentioned, you shouldn't brush your teeth any time around the act because that can open up a way for the virus to get in to your body. You are also at higher risk if you wear braces, have a cold (sore throat), cold sores, or any other sore or lesion in your mouth. Precum and vaginal fluid do carry the virus, but without an opening or sore in your mouth the virus will either die of exposure or end up in your stomach and be destroyed there. There are also enzymes in human mouths that destroy foreign contaminants, and HIV may be susceptible to that.
Swallowing semen, vaginal discharge, blood, or your partner's breast milk (of their cerebrospinal fluid, but I've never heard of that fetish yet...) increases the chances of you contracting HIV, but again, only if your partner has HIV. It increases the chances because there is more fluid with the virus coating your mouth and esophagus.
If you trace the pattern of how HIV is contracted, it's pretty clear that receiving oral is even lower risk (again, unless you have a wound or infection on your cock).
It's not impossible to contract HIV through oral, but it's not common either. The bottom lines are: talk to your partner about STDs, get tested, don't do anything if you're sick, and don't use body parts that have scratches, cuts or sores. Use a condom if you feel safer that way. If you're eating her out you can always cut the condom into a flat sheet and use that as a cover between her pussy and your mouth.
I hope that helps, and have fun. Oral is basically the greatest thing in the universe.
