IMHO....
The 100 psi as nothing do do with a big lost of power or heavy bucking /
shaking, am also sure that the O2 is not the problem, if faulty it would go
on safe mode and the result could be a slight lost of power in certain
condition and for sure a higher fuel consumption, that said....

To check if it's the head or piston / ring, put 2 spoon of engine oil into
the spark plug hole, do the compression test again on that cylinder, if the
compression rise = ring or piston side problem, if it stays the same your
looking at a head problem, this is not a leak down test but it is an easy
"do it yourself test to have an idea.

As for your bucking problem, am pretty sure it's an ignition / faulty ground
/ AFM problem, but like someone else said, if the plug were cooked, maybe
it's time for some new plug wire, cap and distributor.

Regards, Ben