So it "oscillates" even with the power transformer disconnected?
What frequency? Touch your finger to the tip of your scope probe - that is 60Hz. Is the oscillation the same?
I could be wrong, but I really don;t think you have oscillation if the circuit has the power disconnected.
Otherwise, if you do indeed have oscillation at RF, check for an open R168/268.
If you have some sort of sine wave all over everything, and it is at mains freq, then I suspect you are missing a ground. If your scope ground connects to chassis of the amp, and you still have a signal on the ground of the output, the output ground is not connected to the chassis.