[sigeena]

The current iteration of the game allows op-fire even when the unit has used up all its shots in its own movement phase.

However, op-fire is still dependent on the experience and morale of the unit. A "Ready" unit has more op-fires than "Pinned". A Elite, Veteran unit has probably more op-fires than Green.

Previously in the original SP game, you have to retain some fires in your own movement phase for use as op-fires in the enemy movement phase. That may be the historic reference.