"Can I spread genital herpes to my partners mouth if s/he gives me oral sex?"
The exciting and resounding answer to this question is a (slightly qualified) NO! Genital HSV-2 will most likely not be passed to the mouth! It’s 99% impossible (hence the slight qualification). If you use incredibly basic precautionary measures (avoiding oral sex during a herpes outbreak or during prodrome symptoms), you’re home free when it comes to receiving oral sex. Ain’t that the best news you’ve heard in a while? I thought so. Go enjoy yourself. Get down on it.
98-99% of the cases of oral herpes (cold sores) are caused by HSV-1, not HSV-2. (Download the helpful handouts here for more helpful herpes statistics.) According to Dr. Peter Leone, there should be no reason you should avoid receiving oral sex from your partner just because you have genital HSV-2 (of course, avoid oral sex during herpes outbreaks). Even without using dental dams or condoms during oral sex, you run only a 1-2% risk of your partner getting HSV-2 in their mouth because HSV-2 has an overwhelmingly huge preference for the genitals.
What about if I have genital HSV-1? Read this article for more on oral sex and HSV-1.