The OO sound needs a relaxed tongue but your lips should be rounded and pushed forward as though you’re about to whistle.

You will feel your jaw is nearly closed and that the back of your tongue lifts up towards your soft palate – this is the movable part of the roof of your mouth that raises whenever you yawn.

Also, try to anticipate this whistling lip position and don’t let the surrounding consonants distort the vowel in any way. If you slide into the OO position too late, then the vowel will be distorted.