i found a bolt with the letter o an the numbers 9.8 on the head of it under the hood of my 2000 pontiac sunfire. where does that go too?

Who knows, it can go just about anywhere. Who ever worked on it last is who you should be asking.

The number have to do with the grade of the bolt. Bolts are never identified by location.

If you care to know more about bolt grades ----- https://sizes.com/tools/bolts_metric_standards.htm

i let a friend drive my car he called on his way home said he had to pull over the car was over heating an he was gonna put water in it to cool it down just enough to make it home safely. when i went out to my car the following morning i found a bolt on top of my engine with o an 9.8 on it. now i cant get my car to get up over 50mph an idk what the problem could be an hes not around for me to ask plus he has no clue about cars my car starts ok but once it gets warmed up an going i cant go over 50

I would suggest to take it into a garage to have it checked over.

by chance could he have put water where the transmission fluid goes an he didnt put the bolt back in place when he was done an that be where that bolt would go??

Ask your friend but I doubt he removed the bolt.

I'd be thinking damage to the head gasket from overheating or the reason it overheated.