On the flip side, I would think it rare that a wizard somehow ends up preparing spells which are wholly unsuited to fighting an ogre mage at all.
Perhaps you can provide an example of what an "unlucky" wizard might have prepared?
I'm not going to stat out a full 4/4+1/3+1/3+1/2+1 progression for an 8th level wizard. Here's four of 1st-4th that are not bad choices or underpowered for your average party: Lightning Bolt. Charm Person. Minor Image. Mass Enlarge Person. I'm sure we can all imagine the rest of the spell list. They're not foolish choices, depending on the group build, but they're not specifically useful against an ogre mage - at least not in such a way that the encounter turns into a rock-paper-scissors match.
I think assuming a party is going to have a specific loadout of magic items and spells - and then debating the CR of a monster from that perspective - isn't very useful.