While "Rank" and "O-Rank" are both measures of a players abilities, they differ in a few distinct ways:
O-Rank
The "O-Rank" is an overall player rank based on current and prior seasons. Since it provides such a full spectrum analysis of a players abilities, the "O-Rank" is the method used for determining auto-picks in our draft for full Yahoo! Fantasy Sports games. The "O-Rank" is the best measure to use when comparing two players. It should be noted that the "O-Rank" only applies to default league settings.
Rank
The "Rank" value in our Yahoo! Fantasy Sports games applies to the stats that you are currently viewing. For example, on the "Players" page in full fantasy, if you select Season Stats, the player rank value that is displayed applies only to their season stats (or season fantasy points in games use this system). If you were viewing last months average stats, then the ranking value indicates how well the player played on average in a given day during the last month. This rank gives you a sense for how well a player is performing over a limited time period (season, last season average, last week, last month average, etc...).