It is possible for HIV to pass if you are performing oral sex on someone with HIV, but its not common for this to happen.
HIV is not in the saliva so would not pass if someone with HIV performed oral sex on you.
HIV is in the sexual fluids (semen and vaginal fluid), so it is possible if you are performing oral sex on someone with HIV that it could pass if you came in contact with this fluid. Its not a common way for HIV to pass as the mouth has a few defences to stop this.
The chance could increase if someone had open cuts in their mouth, poor dental health or if the person with HIV just received it and had a very high viral load.
If someone had HIV and was on HIV medication so that their HIV viral load was undetectable there would be no risk at all.
To reduce the risk of passing HIV or other STI while having oral sex people can use condoms or a barrier like a dental dam.
Let us know if you have any other questions or concerns.