Arcane Ring is cheaper than Eul’s and it provides Ogre Magi with sufficient mana and regen to keep spamming in the early game. Aghanim’s is the standard item, allowing him to score more multicasts with more damage. Eul’s followed by Guinsoo’s provides Ogre Magi with an extra disable for the team, and BoT is pretty self-explanatory. Most of your items should end there. Blink Dagger is optional if you’re in need of an escape skill.
If the game goes any longer, feel free to get anything you like. You could get MKB to take advantage of increased AS from Bloodlust, or Manta for some hefty images.