First, lets define what oral sex is. Oral sex means using your mouth on a vulva or a penis or an anus. And all of these things can transmit STIs or STDs. They can be transmitted from your mouth to someones genitals or they can be transmitted from someones genitals to your mouth. Although, oral sex tends to have a lower risk of transmitting STIs and STDs than vaginal penetration or anal penetration there is still some risk. And the best way to reduce your risk of transmission is to use some sort of barrier. So, if you were to put your mouth on a penis you would want to have a latex condom on that penis. If you were going to put your mouth on a vulva or on an anus you might want to have a layer of plastic wrap or a dental dam between your mouth and that anus. You can transmit some things from your mouth to someone elses genitals. The real concern though is from someones genitals to your mouth. One of the things the most common things that get transmitted through oral sex is herpes. So, if you have a cold sore on your lip which is an oral herpes you can transmit that to someones genitals even if you actually don't have a cold sore, or at the point of infection where you're shedding where the virus is active. You can transmit your oral herpes to someones genitals. Another thing is that you can from someones genitals to your mouth can be um, a chlamydial infections. So, that can be if someone ejaculates into your mouth and they have infectious semen they can infect the back of your throat. Same thing can happen with vaginal fluid. One thing to be concerned about with anal, oral contact aside from transmission of herpes is hepatitis A which is gotten through fecal matter. So, if you were to put your mouth on someones anus and there were fecal matter on that anus you might ingest that and that may cause hepatitis C. In summary, oral sex although may be a lower risk for transmission you can still transmit STIs or STDs through oral sex. And the best way to prevent transmission or reduce transmission is to use some sort of barrier method between your mouth and the other persons genitals.