OZ is an ounce of shot. Not powder. Powders vary widely by shell manufacturer, shell length because of shot charge weight and velocity.
Going back to the ol lead shot days, a shell loaded with 1 3/8oz #4's or 6's was a pretty standard duck load. 1 1/2oz was a heavy duck load. To compare by weight: 1 1/2oz of lead shot is comparable to 1 1/8oz of steel shot. But because steel is much lighter than lead, there is much more bb's in a 1 1/8oz steel load than there is in a 1 1/2oz lead shot load.
The old shells used to say MAX or MAG to signify their powder charge but much of this was just to boost sales.
Most steel shot loads today simply list their velocity. Most of us are more concerned about velocity than how much powder is loaded in them.