I'm not sure I follow your logic of Xykon's motivations. We know he is a hands-on kind of evil overlord. We see the end of Xykon's torture, when he has started to get creative in his efforts to almost kill O'Chul in new and exciting ways (since he is bored at having 16 hours/day with nothing to do).
We know that O'Chul composed the list of Xykon spells "one saving throw at a time" - i.e. that most of those spells were performed on him. I stand by my statement that it is well outside Xykon's MO to either keep some spells hidden to not give away he has those extra-known-spells feats or that he felt no need to try them out in O'Chul in a moment of boredom.
Grey Wolf