The vowels o and oo share the property of lip-rounding with the consonant w. Make any of these sounds and you will immediately feel your lips rounding. The vowel i, however, does not involve lip-rounding in its production. Sometimes in Anishinaabemowin, the consonant w and the vowel i fuse together (coalesce) to produce an o, that is, they produce a single vowel that has properties of each of them-- the vowelness of i, and the lip-rounding of o. The same is sometimes true of w and a, that is, that they combine to form an o.