You can enter the cart, see what it is, and then reset the cart. It should work. I did it recently when the order took me to a box stock of a patio set that I wasn't going to have time forThis. It makes sense to pull from the backroom first for the majority of items. But, for many OPU items that are in bulk steel, it is way more efficient to just pull from the floor first, in order to meet goal times.
Also, I wish there was a way to view the items in an OPU batch BEFORE you tie it to a cart. When I'm already picking 3 orders and a new order drops for 12 items, whether that batch is 12 cans of soup or 12 pieces of apparel is going to determine whether or not I need to call for help.