If you are HIV-positive, on medication, and undetectable, you have zero chance of transmitting HIV, regardless of what kind of sex you have. If you aren't undetectable there is a very slim chance you could transmit HIV to your partner via oral.
Dr. Thomas Coates, director of The Center for HIV Identification, Prevention, and Treatment Services told Plus that HIV transmission through "fellatio without ejaculation can happen, but it is exceedingly rare.” Even with ejaculation he says, "It’s not ‘no risk,’ but it’s relatively low risk.”
The risk of HIV transmission rises a tiny bit if you're a woman currently menstruating, or the person performing the oral sex has open sores in their mouth.
Nonetheless there are plenty of things you can do to help lower the risk if you or your partner are still nervous. For example: When someone gives you oral always have them spit or swallow any ejaculate. Once swallowed, stomach acid and enzymes in the esophagus kill the virus.