I've noticed when I cast Oath of Desecration on a character with other buffs/debuffs active, sometimes it doesn't apply--or if it does, it doesn't show the status effect icon next to their portrait, or when you hover over the character himself. Could it be a saving throw issue, like in BG/BG2 sometimes your party could resist beneficial spells? Tonight I'll (try to remember to) experiment with it and see if I can find out more.
