S/O is a figure for evaluating the accuracy of a rule in the MassCheck by comparing the hit percentage of the spam corpus against both spam and ham corpa using the equation spam% / (spam% + ham%) where 1.0 means it matches only spam, 0.5 means it matches spam and ham equally, and 0.0 means it matches only ham. See HitFrequencies#The_S.2BAC8-O_Ratio for more detail.