The Irish winger impressed during his debut season for the Black Cats last season but has failed to replicate the same form so far this term.
O'Neill said: "I just think he's been a little bit homesick, that's my thought. James might not go with that.
"There's a difference to telling James to be a bit sensible but I don't think he's gone off the rails or anything. My concern is him playing well for us. If there's something off the field, then on a serious note I have to look at that."
McClean's made his debut for the club last December having previously been a regular for the reserve side prior to O'Neill's arrival.