Sunderland have been forced to shuffle their defence in the opening two months of the season, and despite only sitting in 13th place in the Premier League table, they have continued to remain solid at the back.
O'Shea told the Shields Gazette: "I think defensively we've been strong despite not having had a settled back four so far.
"I think that's all credit to the players involved. You've had Titus Bramble and Danny Rose, for example, slotting in without disrupting things.
"That's what the manager wanted this season – to improve the squad in defence as well as attack, and the signings he has brought in have done that."
Sunderland will be hoping to continue to keep it tight in defence when they travel to Manchester City on Saturday.