The /o:/ vowel shown in the list of vowel semments (§2.1) is not a basic phoneme of this language. Instead, it represents underlying //au// in certain combinations involving Anaphoric demonstrative root //-uba-// (chapter 7), principally that involving the punctual form //a-// of the class prefix of the ANA noun class (4.7). It is also sometimes used in the texts for the rounded trail-off vocalism in the elongated interjection pronunciation of some words after phonological rule P-39 applies.
In the demonstrative combinations just mentioned, the actual phonetic realisation of //au// ranges from something like [aw] to [ow] or [o:], and does not usually have the steady-state quality suggested by the transcription. We generally normalise transcriptions, as in /o:-'ba-ni/ 'that one[ANA class]', rather than aiming at a close phonetic transcription for each individual occurrence.