You probably already know this...but perhaps the sequence of bolts used when reinstalling the timing chain cover was mixed up? All of those are different sized bolts and have to be installed in the exact same spot, IIRC.

The first thing to do regarding the oil leak is to determine exactly where it's coming from on the sucka .

I've never had the occasion to change my O2 sensor, but I recall reading that one of the things that it definitely doesn't like is sniffing a lot of burnt coolant. Perhaps just change it? The other thing I was thinking re: check engine light is that something simple might have a loose connection (or not be connected at all) a la ICV or MAF. Of course, losing oil pressure can't be helping anything very much.

best, whit