I heard a preacher say one time that Onesimus ended up becoming an elder of the church. Is that not speculation and conjecture?
Ignatius, a Christian who wrote at the beginning of the second century, to the church in Ephesus said, "Seeing, then, that we have become acquainted with your multitude in the name of God, by Onesimus, who is your bishop, in love which is unutterable, whom I pray that ye love in Jesus Christ our Lord, and that all of you imitate his example, for blessed is He who has given you such a bishop, even as ye deserve [to have]." Ignatius also wrote, "The bishops, Onesimus, Bitus, Damas, Polybius, and all they of Philippi (whence also I have written to thee), salute thee in Christ. Salute the presbytery worthy of God: salute my holy fellow-deacons."
Since Onesimus came from Colosse and was a slave to Philemon in Laodicia, it is sometimes assumed that this is the same gentleman since Ephesus is not that far from the other two cities. Catholic writers are the ones who promote the idea the most. However, it is not a necessary assumption because names are not unique.