I'm not an expert on olives but I'm doing exactly what you're describing in Z8b Oregon. I planted two olives to provide privacy screening above a 6' fence. I've looked at a ton of trees in garden centers this spring and they've all had skinny trunks and were staked, and my trees are no exception. I'll keep them staked as long as they need it, but if the trees cannot stand after the first growing season with the original stake, I suggest using 2 stakes on opposite sides, further from the trunk. Allowing for a fair bit of movement with the wind will help the tree gain trunk caliper faster, and using two stakes with fairly loose ties will allow this movement without allowing the tree to list one direction or the other.

The trunk will get thicker but olive trees like sprouting new branches from the base so you will have to do regular pruning if you want to keep them single trunk. Also do follow above advice on getting rid of the tight green tape as soon as possible.