Beacuse you set MCT and then select A/THR off, the thrust is fixed at the high value.
Pulling the ALT selector for OP DES as correctly stated will make the aircraft descend ONLY AFTER the speed drops below the selected target value, and the A/C will dive to maintain it.
OP DES is a speed mode, remember. Technically speaking, if the correct drill sequence is followed, the question makes no sense.
You can pull as soon as you want.