The service limit for these engines is 180 (compression). If you're getting 150 with the front O2 out, what were you getting with the O2 in?

Also,, the third gen S cars had a "pre cat" under the hood S well as a normal cat under the car.

I'm going to guess the precat on your vehicle is factory original. I don't remember the configuration, but please make sure both cats are out of the system before you give up on the exhaust leak theory..

In addition to the oil control rings, the valve guide seals fail and pass oil into the cylinders.

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The first oxygen sensor should be before the first cat. The burnt oil ash builds up and plugs the cats, this creates excessive heat and melts exhaust valves. Saturn S series had notoriously problem rings from the factory if oil changes were extended. Mt experience with some high ash content engine oils is that if you change your oil when the oil darkens they work fine. However if you change oil by the odometer or oil change minder the engine will turn tho sludge and the problems begin.
It might be time to just go through it completely. I have a couple Ecotecs sitting that need rebuilt.