Move and then attack. This means all the units represented by that tile move first (or don't move at all) and then all get a chance to attack. If any one piece is used to attack, all the other ones that haven't moved can no longer move.
You are not required to move all OL units first, but it is "almost always" most efficient to do so. You can, e.g., move 1 unit and immediately attack with it. All others units in that tile group then lose their movement points. After those free movement points are used/gone, the OL can spend gems to continue movement: 1 gem to move 1 unit from that tile group 1 space, up to an extra movement limit (per unit) equal to the tile's base movement value.