It was on O2 sensor that kicked the Check Engine light into life.
That's pretty common.
No reason was found for the oil leak. They put some dye in the oil. I'm to drive it and bring it back in a month for a regular oil change (or sooner if I notice a problem) and they'll use a black light to see what they can see. Needless to say I'm driving quite cock-eyed, one eye on the road and one eye on the instrument panel looking for an oil light to go on. Diagnostics and repairs were about $330.
If your oil light goes on, the engine is on the verge of self-destruction and should be shut off instantly.
Just make sure when they fix the oil leak, that they use O2 sensor safe silicone sealant.
Do you want to hear my blown seal joke??
An eskimo brings his snowmobile into the shop and it's leaking oil. The mechanic looks at it and says "it looks like you blew a seal". The eskimo says "no, that's just frost on my mustache".