Oh (or /O/ as it is written in IPA) is the wonder vowel. That's what I call it anyway. It is the vowel that comes out of your mouth when you see the grand canyon for the first time, step into a magnificent space - say a Gothic cathedral, experience your first takeoff in a jet, or are in the throes of passion. In that sense, it is a very high vowel, and by that, I don't mean acoustics.
What happens when you emit a genuine and clear Oh? Two things simultaneously: The jaw and face both open up. What do I mean by the face opening up? The 'mask' of the face- the area around the eyes - becomes very alive and the muscles of the face feel and look sculpted. The eyes themselves are wide, big as saucers. And the jaw? It retracts a little father than it does for Ah. These two actions aren't something you can fake or ape. They are motivated by intention. You have to be like a kid again, out on the playground, watching a friend do something daring or silly, commenting and pointing with your finger......"oooooohhhhhhh!!!!"
Take a moment right now to do it. But if you do: Go find a mirror. It's easy to think you are 'doing' it, when, in fact, you aren't 'doing' anything at all. In this case, seeing really is believing. If you are the thinking type (the face will want to close), go watch movies with people exhibiting real joy and do likewise. If you feel like a fool or vulnerable? That's part of being open. You'll get over it. You'll feel so alive that you'll stop being self-consious. And that is a very good place to be. You'll be full of yourself in a good way.
You must be a kid again to enter the kingdom of heaven.(ok...so I am paraphrasing)
What happens in the body while this gleeful Oh takes over? The spine lengthens in two directions (which you can feel by holding a hand on the nape of your neck), the chest lifts and opens, while the breath is suspended, the latter synonymous with inhalare la voce. Get used to this feeling! Stay in it without effort or strain.
Why is the lengthening of the spine important? For one thing, it is a indication that the ear is actively processing, that is, listening to, higher frequencies (read my posts on the work of Dr. Alfred Tomatis for more information). They make singing possible. Let's be clear here. Can you shove yourself into a position where you mechanically straighten your spine and, in doing so, 'open' your ear? Nope. Sorry. It doesn't work that way. It has to come from within.
Real joy and wonder has 'consequences.' This is why it can be damn hard to sing when something bad has happened to you. The ear shuts down, the body contracts, the shoulders hunch, the face closes and darkens.
When your Oh holds you, staying clear and true without degenerating into a diphthong (the jaw will have moved), the face staying open without becoming fixed or stiff, you'll then be able to practice OO. It's just a little more condensed.