olympus om-4 metering with depth of field preview button?

Hi, I just bought an Olympus Om-4 and I’m new to the auto exposure/metering feature for film cameras. Does anyone know if I need to press down the depth of field button on the lens to get the proper light meter reading or will it do it automatically? Confused because light meter level in viewfinder doesn’t change when I adjust the aperture unless I hold down the depth of field button. Thanks!