Ozzie says Lavos is already summoned, but Magus is in the midst of summoning him. I think Ozzie merely felt the massive energies of the spell getting started, and, given his confidence in Magus' abilities, assumed that Lavos was as good as summoned at that point.
Then again, the thought that he was fully summoned at some point ties in to Belthasar's observation of Lavos disapearing. I still think that Lavos' actual summoning occured during the battle with Magus.