does it make any funny sounds when it turns over? I'm just thinking, what did someone do before you? most extreme case, someone took the pistons out to re-ring and never put them back. that will give you zero compression for sure.
If I were you I'd pull the head. not too much work and in my opinion you should check what's hiding there. same goes for the oil pan. definitely wouldn't hurt to have a look at the crank, cam, and also what is in the bottom of oil pan (pieces of metal maybe?)
