The risk of getting a STD if you engage in oral sex is smaller than having unprotected (anal) intercourse, but it’s not safe.
If your lover has genital herpes and you stimulate the genitals with your mouth you can get a cold sore on your lips. Or if your lover has a cold sore on the lips and gives oral sex you can get genital herpes. AIDS, gonorrhoea and syphilis can also be transmitted through oral sex.
You can protect yourself by using a (flavored) condom with a blow job or by using a dental dam when you go down on a girl. You can make this more exciting for the receptive partner by putting some water based lube on the inside of the condom or the dental dam.
If you engage in oral sex without using a condom or dental dam don’t get semen and/or menstruation blood in your mouth, decreasing the risk of contracting AIDS.
Washing the genitals before or after oral sex doesn’t help when it comes to STD’s.