Hi, welcome to GoY, Olea europaea, or olive tree, wants to grow into a tree 3oft high, so will eventually need some lower branches removing, if the tree doesn't shed them itself, they're pruning group 1, which means they should be minimally pruned, only removing dead, diseased or crossing branches, pruning the top branches wont make much difference to the amount of leaves on lower branches and could permanently damage the tree, Derek.

Are you trying to keep it as a topiary? If so, you can give it a moderate heading back once it gets a little warmer. I wouldn't take off more than 1/4 of the total foliage. To keep it full to the base, prune it so it is narrower at the top than at the bottom.