This contrast is between a short vowel which cannot occur in an open syllable and the schwa which is always unstresed. In theory there could be a contrast in word initial position; if there was such a word as obbout it would contrast with about. In practice the only pairs that could be found in the dictionary were contrasts involving weak forms.
There is no point in calculating a density value.
on an on 'un oz as
Credit: John Higgins