Hi,
I know its quite late to answer this but I thought it can hep others.
If you want to check material availability during order creation in same way as it does at order release, in OPL8, set Reservation/Purch.req. indicator to 3 i.e. immediately reservation creation.
If you not set this, then system will always show All checked materials available during order creation irrespective of what settings you maintain in OPJK.