97 Olds 88 won't start

Hi. My dad's oldsmobile has 120,000 miles and is a 3.8 liter regular engine. It turns over but won't start. He made it home with the overheating lamp on, but when he went to try and start it again it wouldn't go. No smoke coming from exhaust. I took the coolant cap off and it was very low. We put a lot more in and then had to add more later. We filled it up and tried to start it with the cap off but it didn't spit up. The oil looks reasonably clean. I pulled a spark plug off and there is spark. I changed the fuel filter and pressure regulator. One other thing, there seems to be a lot of oil around the head covers. I am out of ideas. Can you help?
I just checked it again and the coolant looks like it's gone again. There's no puddle under it? Weird.