While the risk of HIV transmission has been considered a theoretical risk, in reality oral sex is highly unlikely to transmit HIV.
For a man receiving oral sex there is no risk of HIV transmission if the skin of the penis is intact.
While HIV can be transmitted in vaginal secretions the exposure to these are very low for a person giving oral sex to a woman. Saliva also contains enzymes that kill HIV.
A person giving oral sex to a man again has a low risk if there is no ejaculation or precum present. Volumes of precum is usually quite low and the enzymes in saliva would degrade any HIV present.
Ejaculation into the mouth poses a very low risk of HIV again due to saliva as well as stomach acid killing the virus. Spitting out semen and washing the mouth also removes virus particles.
If there are any ulcers, cuts, sores in the mouth or recent dental extractions it is best to avoid oral sex with ejaculation.
Use of condoms or dental dams for oral sex with both men and women essentially eliminate any risk of HIV being passed.
Good dental care is recommended such as daily flossing and brushing of teeth.