OM lenses don't have an auto diaphram lever as you can't switch to manual as you can with some Soviet cameras. The only real use for such a lever is to provide DOF preview and there is a separate button for that. The point of an auto diaphram is that you don't have to manually stop down to the taking aperture for metering. The camera meters with the lens wide open then stops down for you when you press the shutter. The result is a much brighter viewfinder image and faster reactions. Virtually all modern cameras are set up like this.

David.