O-Ranks are based on the player's numbers over the last three years. That's why guys like Milton (who was injured for most of last year) and guys like Overbay (who got about 260 major league at-bats prior to this year) are ranked so low. Miguel Cabrera, Derek Jeter and Kaz Matsui are other examples.
Ranks are based soley on the player's performance this year (or in the last month, or last week, according to the filter you have selected). The ranks aren't perfect, but they can be used as a quick indicator of how the players have been doing.
Used together, if an established player (who hasn't been injured for long periods) has a high rank and a low O-rank (for example, Beltre, who has a rank of 46 and an O-rank of 144), it might indicate that the player is playing over his head (sell high). Conversely, a guy like Sheffield, with an O-rank of 11 (great previous performance) and a rank of 181 (poor performance this year) might indicate a buy low opportunity.
You have to take the rankings with a grain of salt, but I won't jump on the bandwagon and say that they are completely useless.
Hope this helps!
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