At times we all get a tune or a phrase stuck in our thoughts. This can be irritating but eventually the thought goes away. In OCD the thoughts and images can persist for hours each day, may be quite repugnant, and can cause significant anxiety. The thoughts may involve violence to others or distasteful sexual images or frightening images. Some evidence suggests that becoming frightened by unwanted thoughts can lead to them recurring.

Deep Knowledge Reference
Purdon C. Cognitive-behavioral treatment of repugnant obsessions.
J Clin Psychol. 2004 Sep 10.