Short Answer:
Oral herpes or cold sores are usually caused by HSV-1.
Long Answer:
Cold sores are caused by the herpes simplex virus. You’ve probably heard of herpes before, although, there are actually two types of herpes simplex virus, “type 1″ and “type 2.” Herpes simplex virus type 1 (HSV-1) commonly only infects those body tissues that are above the waist, and it is HSV-1 that causes cold sores in most cases.