o.

Headword: 
o.
Principal English Translation: 

(indicator of the optative or the preterite -- also an exclamation)

Alonso de Molina: 

o. es señal de optatiuo, y tambien delos preteritos, e interjection del que esta afligido yhaze exclamacion.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 74v. col. 1. Thanks to Joe Campbell for providing the transcription.

o. he aqui. aduerbio.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 74v. col. 1. Thanks to Joe Campbell for providing the transcription.

Attestations from sources in English: 

If this is a preterite marker, and no other subject prefix follows, this is an indication of the third person (singular or plural, which might be sorted out by examining the verb ending).