The present letter is not dated and it can only be said that it was written at some point during the episcopacy of Ambrose. Palanque conjectured that Orontianus was of Eastern, possibly Syrian origin, but his arguments - an alleged knowledge of Greek by Orontianus, and a fact that he must have been away from Milan to receive letters from Ambrose - are unconvincing. Mazières (1973: 52-54) discussed also an old thesis of the Maurists that Orontianus might have been of Jewish origin - although he rejected the passage originally used to claim this (Letter 20.15), he traced other passages and recurring topics (concerning the role of the Jewish people in the history of salvation - in Letter 18 it is a conversion of a weak soul, "like Israel according to the flesh", par. 19) referred to the personal experience of Orontianus (here in Letter 20 see the remark in par. 15 that Orontianus "started from the Law") that made this conjecture possible.
For Orontianus ecclesiastical status see discussion in [1874].