The vowel ŌO

The vowel ŌO requires the highest arching of the palate, which in itself ensures that the sensation of a cupola is achieved. Since a sufficiently arched palate is also the requirement for a correctly formed head tone, this vowel will serve for practicing the head tone as well as an initial vowel for the practicing of all other vowels.
Most students (and unfortunately also alleged singers) face difficulty relaxiing their jaw for the vowel ŌO, because a certain disproportion occurs when the vowel is sung well technically. While the lips are relatively closed - their distance is affected to a great extent also by the tone pitch, the jaw should be relaxed and quite open. Take care that the upper lips and the upper part of the face is not flabby.
Usually, when the students articulate the vowel ŌO, they also clench their jaws. When they try to relax the jaw, they in turn fail to articulate the ŌO.
A vocal imitation of an owl's hoot through which you achieve a certain "barrelled" hollowness guarantees the maximal arching of the soft palate. D124a D124b
Since this necessary space can only be achieved by positioning the chin properly, I recommend doing a preparatory exercise with the children first, during which they relax their lower jaw and place their chin slightly back.