Column XO is an indicator in Prashant Shah's book Trading The Markets The Point & Figure Way. It counts a number of Xs and Os in every column for the length period. Both Xs and Os are calculated separately. Then, both results are divided by half the number of columns which is set by length.
I personally don't find this indicator very useful, because all it can offer is very clear in Point and Figure charts. However, it was designed to give some information on volatility as well as direction.