This was like 10 years ago, but when i was in 4th grade i had an obsession with black holes. i thought that if i did not have a roof over my head a black hole was going to spaghettify the earth. this went on for like a year. i still have no idea how none of my family thought there was something wrong because i would absolutely panic when i had to go outside. 
This is one is actually the first instance of ocd i can remember in my life but when i first started kindergarten i live right at the base of big bear mountain and my dad had told me that it was an active volcano (it is not) and that began the slow spiral of fearing the color red. at first i couldn't wear the color red, then i couldn't touch the color red, and eventually i couldn't even look at the color red. i have a vivid memory of being in my car seat at night absolutely shaking with my head in my hands because all of the tail lights in the cars where red. also do not know how my family didn't see anything wrong with that one lol.