He's not one now?
Nope. Calling audibles at the line has very little to do with being an OC. An OC has a large amount of responsibilities. Do we know that Manning can design an offensive system utilizing all of the different talents he is given? No one could know that based on him being a great QB. Could he help a young QB learn how to prepare for Sundays? Sure. But I don't know that we can say he can do much beyond that with a great degree of certainty.