First off, don't ever try to understand Yahoo's ranking system.
From what I understand, the O-rank is more of a multi-year average and the rank is his ranking for this year. I used to think that O-rank was the original rank at the beginning of the year to compare to the more current rank but they both change during the season. If someone was just letting the computer pre-rank their players and choose them for them, I would assume that they would go by the O-rank number. Confused yet??