Short Answer:
Yes, it is very possible that you could contract genital herpes from oral sex.
Long Answer:
Cold sores are caused by herpes simplex virus type 1. If you have oral-genital sex with someone who has a cold sore, this virus can give you genital herpes. The likelihood of this is particularly high during a primary outbreak. Washing with soap and water after a sexual encounter can help decrease the risk of transmission. Barriers such as dental dams can be used during oral sex to help protect against herpes and other STDs as well.