Multicell overvoltage protection with op amps, how to get rid of oscillating behavior?
It sounds like the real opamps you used in the circuit are going unstable. This could be due to a phase margin that is unacceptable. You basically have two series opamps and a MOSFET in a control loop and this can easily go unstable. A single opamp with local feedback can easily be made to go unstable (how you make an oscillator) so your configuration is likely to be problematic.


  I put a low pass filter at the gate which improved the situation,


This can be a useful trick to remedy unstable circuits. It’s called “introducing a dominant pole”.


  But can the source be addressed directly?


The “source” is the target circuit that you have evolved and is a common problem facing analogue circuit designers. There’s probably nothing much you can change in your circuit so, you add a dominant pole!

One thought, you should probably add A resistor in series with the source of the MOSFET. This can reduce gain dramatically and needn’t be a hindrance to correct circuit operation. Decide what value you could tolerate when trying to balance cells and put it in place. It’s probably a good idea anyway because you don’t want the MOSFET turning fully on under any circumstances.

One last thing. Don’t use 741s; they are such dinosaurs and have poor performance compared to more modern devices. Even the LM324 (despite its age) is a superior choice.