Imagine the feeling of having a song stuck in your head. Now imagine that instead of "It's Raining Men," it's the thought of murdering your best friend. In graphic detail. Over and over again. You're not mad at your best friend, and you've never done anything violent, but it won't stop playing.
You probably feel kind of shitty just reading that. But that's what the "obsessive" part of OCD is like: intrusive, unwanted, disturbing thoughts that won't go away. No one seems to know what causes them, although it might be miscommunication between parts of the brain or something faulty in its error detection circuit.
"Ohhhh, you don't want to feel debilitating helplessness. My bad."
The thoughts aren't always about you doing bad things, but they're never pleasant. Most obsessions are based on deep fears -- "What if I or someone I love gets sick?" -- or basically the worst things one can think of, like blasphemy, racism, suicide, murder, rape, contamination, animal abuse, cannibalism, torture ...
"Excuse me, I have to go rape some video murders."