Well no, those areas were still massively Aramaic. There was a significant Arab presence though.
Anyway, to convert Seljuk to Christianity while making Osman a Turkish name, just have an Arab Christian priest or merchant named Uthman go to the Oghuz Turkic territories.
That's reasonable enough, I guess, though it still doesn't get over the obstacle of having the Turks settle Anatolia without going through Persia first.