While the +5 item and the maxed ranks is reasonable, I would not think it's assumed at 6th level. I'm not even sure what premade items exist grant a Spot bonus.
Eyes of the Eagle from the PHB
But to succeed, you first have to be close enough.
To be a threat, the Ogre Mage has to be close by.
A four point advantage in the ogre mage's favor is pretty good.
Last time I checked, 12 - 10 = 2
I have a question. How can you consider the 60% chance of success on a grapple check from the Ogre Mage vs the fighter good, while considering the 70-75% chance of success from the wizard casting Glitterdust bad?
Why would an ogre mage who is invisible do anything OTHER than stroll casually right into the middle of the party, not adjacent to the rogue with the maxed out Listen skill, but instead adjacent at an unknown location next to the easily grappled wizard who does not have a maxed out Listen skill?
Grappling the wizard means you are denied Dex to AC, lose invisiblity, and can be sneak attacked by the rogue and murdered by the fighter. Cleric will either cast a spell on you or bring out a mace. Furthermore, when the Ogre Mage starts the grapple, he spends his first round establishing a grapple and he can't deal damage until his second round.
Also, there are several familiars with spot, listen, blindsense, and scent which may make sneaking up on the wizard harder than it sounds.
The mage would not purposefully include party members in the area of glitterdust, which means the ogre mage can purposefully stand near the party when it's safe to do so.
Ogre mage occupies a 10 ft square. Glitterdust affects a 10 ft square.
If the Ogre Mage simply stood next to someone, the Wizard would just have to keep a 5 ft distance between the edge of the Glitterdust spell and the character.
Now imagine the ogre mage readying cone of cold to cast if the mage casts any spell at all. Hilarity ensues. No glitterdust, possibly no mage, no full attack from the fighter. Maybe the rogue can flank him.
I'm imagining. Wizards can cast Glitterdust more than once a day. Ogres mages can only cast Cone of Cold once per day. Do you see where I am going with this?
If you kill/knock out the wizard with the Cone of Cold, things get tougher for the party. Unless the Rogue has a sack of flour, Enlarge Person, or Faerie Fire.