Why was the head gasket replaced by the previous owner?

Since compression is low on all cylinders, valve timing may be incorrect ( check camshaft and timing chain ) Also check valve lash , it may be too tight. This still matters on a engine with hydraulic lifters / hydraulic lash adjusters. Hydraulic adjusters still need to be operated in their proper range otherwise the valves can be heald open.

Does the car have a catalytic converter? It may be restricted.

A oxygen sensor ( lambda sensor , o2 ) generally does not have enough control to make the car run that badly. O2 sensors are more to trim fuel mixture.