Actually on some cameras this is a real problem. But it is not the diaphragm itself that causes shake, but the mechanism that actuates it. I don't have an OM, but some of my older cameras like the Edixas have a very sharp shutter button "break" when tripping the shutter due to the triggering of the aperture plunger. On the older Edixas you can actually avoid this by slowly depressing the button so that the plunger is triggered before the mirror is released. The jar from releasing the aperture plunger feels sharper to me than the mirror slap, and it is non existent though if one uses an early CZJ lens for instance which blocks the plunger from releasing.