The Saints have had an excellent start to the season, as they are currently third in the top flight, while striker Graziano Pelle has led the way for Ronald Koeman's side with five goals for the club following his summer move from Feyenoord.
O'Shea told the club's official website: "We will also be very wary of how Southampton have started. They have had numerous changes and a new manager but they have carried on from last season.
"The new players have settled in very well and I know Shane Long quite well from Ireland, he has been in and out a little bit but he could be starting as well.
"We will be familiar with one or two faces but we will also be very prepared and the manager will have talked to us about the new lads too, so there are plenty of threats to think about."
The Black Cats face Southampton at St Mary's Stadium later today.