As someone who (now after about a year of therapy a while back) has my OCD managed, let me tell you what my brain still does with a moment of free time:
Visualizes the worst possible scenario of any situation I’m in!
Standing in line to get coffee: What happens if I blurt out a racial slur randomly in here? Or what if someone walks in with a gun? What if someone put poison in this batch of coffee?
Walking with my kids to a park: What if that van over there is about to kidnap my kids. What if my oldest pushes my younger one out into traffic? What if one of them falls off the playground and gets permanent brain damage?
Sitting in bed at night: What if that weird feeling in my stomach is cancer? What if I left the heater on too high and it starts a fire that burns down my house? What if a giant sinkhole opens up beneath my house and swallows us whole?
All that being said, I used to spend hours compulsing to try and make the anxiety these thoughts cause go away but have learned to better accept them for what they are.