Is oral sex "safe"?
Oral sex carries a lower risk of HIV transmission than anal or vaginal sex, but it is not risk-free for other STIs. Several infections are efficiently transmitted through oral-genital and oral-anal contact. The actual risk depends on the specific infection, the sex acts involved (giving vs receiving, and which body part is involved), the presence of sores or other lesions, and oral health.
HIV and oral sex
The risk of acquiring HIV through oral sex is very low but not zero. The CDC and BASHH consider it a "negligible" risk rather than zero because there have been rare documented cases. Factors that increase the risk include ejaculation in the mouth, oral ulcers or bleeding gums, and genital sores. The risk of giving oral sex (fellatio) is higher than receiving it, and performing oral sex on a vulva (cunnilingus) carries an even lower risk.
Gonorrhoea and chlamydia via oral sex
Gonorrhoea of the throat (pharyngeal gonorrhoea) is common and is acquired through giving oral sex to a penis. Most pharyngeal gonorrhoea causes no symptoms. Importantly, it is often missed because standard home test kits do not include throat swabs — if you have oral sex, make sure your sexual health screen explicitly includes a throat swab. Chlamydia can also infect the throat, though this is less common and less clinically significant.
Herpes and syphilis
Herpes (HSV-1 and HSV-2) is readily transmitted through oral sex — HSV-1 (traditionally associated with cold sores) is now the most common cause of genital herpes in many countries, acquired through receiving oral sex from someone with oral herpes. Syphilis can be transmitted through oral sex if one partner has an oral sore (chancre) or secondary syphilis lesions in the mouth. Condoms and dental dams reduce but do not eliminate this risk.