Caecilius wrote:I think it's semantically quite clear: "The son ought to wait [i.e., it is fitting for the son to wait] until his father returns."
But as you say, oportere takes the accusative and so would exspectare, I suppose there could be ambiguity in that it could well be "It is fitting to wait for the son until his father returns", yes. I might be wrong on this note, though. I normally wouldn't think that there's any rule governing oportere when it's used as an intransitive, but maybe there's something else that could be mentioned to help distinguish further. At the moment, though, to me there's nothing (technically) wrong with either.