I looked at my cousin's d16y8 earlier and I found that the coolant resivour was full of oil and the radiator had oil in it as well, any idea of what it could be, I was thinking maybe piston rings and head gasket but not sure.

 

You won't really know until you pull the head off.

Could be just a head gasket or it could be something more.

 

Could also be caused by a cracked head or block.

 

pistons rings won't cause coolant and oil to mix.
One of the following:
* cracked cylinder head
* cracked cylinder sleeve(s)
* bad headgasket