The mother brine is the liquid the Kalamata olives are floating in. I
assume the olives in the deli are available in a large container of
some sort and that you or the counterperson scoops out the desired
amount.

So you can retain the liquid and keep the olives in that or drain them
and store them in olive oil. The only disadvantage of the olive oil is
that it will solidify under refrigeration, but a short time at room
temperature remedies that.

Carol