I use mole grips (v hard to do with pliers) across the widest part of the O Clip, i.e. so they are crushing the O Clip where it's been pinched. As it is pushed back towards the fitting, the O clip widens.
Hard to explain. If you took the O clip with its bulges to the three o'clock position (or 9 and 3 if it's a double pincher) then you'd be applying force inwards at the 9 and 3 position using mole grips . As they move inwards, the six o'clock and 12 o'clock sections move outwards, making it easier to remove.
Vin