The amount of something (ma-) in a container is decreased, less.
Mabak danaw byas ko do lata.
My rice is already less in the can.
The amount of something (om-) in a container becomes less, decreases.
om- mabak, omin- minabak
Minabak danayaw inapoyaya do kaldero.
The cooked rice in the pot has already become less.