I'm not sure which symptom was actually the first, but my OCD first reared its head when I was about seven or eight. One of the first things I remember was being haunted by this horrible vision of my mom being burned to death in a fire. I used to visualise her charred skeleton, and this caused me a lot of upset. Thankfully, it never happened, and my mom is still very much alive to this day.
But I went through this spell back then, which ended one day when I broke down in front of my older sister, and told her all about it. It didn't bother me after that, and I'm not sure that being haunted by ficticious black thoughts are OCD symptoms, but I have a feeling there's a connection.
I had other symptoms around that age, too. I can remember my best mate at school at the time claiming that he once drank blood from a crab, or something daft like that. So I worried that he was going to die as a result of that, even though he was fine, and some time had passed since this supposed thing he did.
I also used to flick imaginary balls of plasticine (or something) off my thumbs while I was sitting in the classroom. Whether I did this an even number of times or not, I don't know, but I used to do it. I also went through a phase where I had a horrible fear of death, like I'd ask my mom "will I die if I do/don't do this?"
So there's quite a few symptoms from my childhood, but I can't accurately pinpoint which symptom was the first.
Steve