Does the combat log say the buff was applied, rather than fumbled?
In a quick test I could easily get 6 status icons to show up on a character, but that was in the initial fight of the game, so there could be issues with more characters and opponents involved (I have not played much in the last couple updates).
Buffs and debuffs that are direct opposites (like hasted and slowed) will cancel each other out (both statuses removed). Oath of Desicration and Weak will not, since Weak also has a Strength debuff, in addition to cancelling out the 50% damage bonus from OoD.