I can further clarify...
O-Rank, or Overall Rank, is a rank that combines statistics from the past few years, combined with some hand-tuning, to try to provide a generally useful rank.
The other rankings are purely statistic-based for the time period you are viewing (last season, last month, last week).
In both cases, rankings assume our default 5x5 categories, and our default public league sizes. O-Rank is more subjective; you could consider that a good or bad thing. For example, no rookie can possibly score well in the previous seasons season ranks, because they have no stats. But the O-Rank may hand tune to elevate them. In the end they are just different, and both hopefully useful, tools; neither one can tell the full story.
I guess it hasn't been said yet, either: if you have looked at the statistical rankings in the past, and found them dubious, take another look. I pushed our new ranking code for baseball last week sometime, and I think you'll find the statistical ranks to be more in line with conventional thinking now. It will be described in some detail on the Yahoo! Sports Blog
sometime soon; that post got swept up in all the March Madness hubbub