The chances of getting genital herpes this way depend on 2 things:
1. If you already have the cold sore virus on your mouth yourself
2. If the person who is giving you oral sex has an active sore or not
For safer oral sex, using a condom (for oral sex on a penis) or a dam (for oral sex on a vulva or anus) is still safer than nothing at all. A dam is a thin, square piece of latex that is placed over the clitoris, vulva, or anus. You can also use a non-lubed condom by pulling off the ring and cutting along one side to make a rectangle; or, use a small piece of plastic wrap (preferably non-microwaveable wrap because it is less porous). And just to be clear, herpes can be transmitted by either the giver or receiver of oral sex. It is possible for the person giving oral sex to get herpes if their partner has genital herpes and a sore is active or there is viral shedding. It is also possible for the person giving oral sex to give herpes to their parther, if they have an active herpes sore or viral shedding on their mouth.