"One day" can be in either the past or the future. "Once" by itself (ie. a comma follows) means "one time" and tells me the next statement will be in the past. However, if you attach it to the phrase, the present simple tense indicates the time of a future event, and the simple past obviously means the past. In this case, it means "at the exact moment..." "Once I saw her, I told her the news." "Once I see her, I'll tell her the news."