I don't know the specifics of the european olive, but any tree can be trained. Let it grow for a season and see if any straight suckers come out, or select the most upright branch now and tie it to a bamboo stake or that sort of thing to bend it more upright. It will adapt to this new shape after a seasons growth. With a tree that small, you have plenty of time to work with it.

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Dave
Even my growlights are getting restless!

Yes...if there is a bud next to the cut, it will grow out and up, and can be encouraged to grow straight up if it doesn't do this on its own. You may or may not see a visible bud. You might want to start traiining a limb below the cut, if there is one that has potential (i.e., isn't rather horizontal,) for insurance.

Ahhh, wait...I just reread your initial question...You're just worried about the tree getting taller? You see, open center is a term for the way a peach is grown. The central leader is cut and the tree trained to grow several widely spread leaders...the open center allows light in to heop the fruit ripen.
Most nurseries eill cut the central leader of a tree in order to make it fuller, and thus more appealing to customers. In this case, the tree normally sprouts TWO new leaders, and in general, energy is redirected to make the whole tree fuller. A "central leader" type tree jsut means it has one trunk. If you just want one trunk, you may need to follow what I said above. Plus, if the tree buds more than one new leader, you'd need to prune to leave the best one only.
Please let me know if all this makes sense...like I say, I thought you were wanting one dominant central leader. The tree will still grow pretty much as tall as it would have, whether you train to one leader or not.

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Dave
Even my growlights are getting restless!

Good luck, Daniel, Happy Growing!
You should check out the rest of this site, and visit often! Everybody around here is really friendly, and fun.

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Dave
Even my growlights are getting restless!