The 35-year-old Republic of Ireland international is a veteran of the club's increasingly remarkable fightbacks in each of the last four seasons to stave off the spectre of relegation from the Premier League, and had high hopes of playing his part in another.
However, Wednesday night's 1-0 derby defeat at Middlesbrough left the Black Cats 12 points adrift of safety with just 15 to play for, and knowing it could be all over on Saturday if they do not beat Bournemouth and results elsewhere go against them.
Asked if this is the toughest position the club has been in, O'Shea said: "Without a doubt
Games are running out.
"But while we still have that chance, we have to perform and do the minimum, which is to have that work-rate, effort and attitude.
"It was there on Wednesday night, but it's that bit of quality also we need to get in front of teams and finish them off
We're going to need that against Bournemouth because they've shown they have plenty of quality too."
The statistics paint a damning picture - Sunderland have won only five league games all season and just one since December 17, and have collected a meagre seven of the last 48 points on offer.
Manager David Moyes, owner Ellis Short and on the final whistle the players, all found themselves in the firing line at the Riverside Stadium as disgruntled fans voiced their frustration, and O'Shea knows there is only one way the and his team-mates can answer them.
He said: "Yeah, look, rightly so
Nobody's happy at the position we're in.
"You can see attitude and how we're working in training, what we're trying to do
We're trying to get some results in games and it's extremely frustrating.
"It's very simple: whatever team the manager picks has to stand up and be counted
Whatever performance we give, the result is key to keep ourselves alive.
"You have to find that resolve from somewhere to get that win
Ultimately now performances are not the be-all and end-all, it's the results we need to give ourselves a chance
It's a huge task."
The visitors started brightly, but were rocked by Marten de Roon's ninth-minute strike, which ultimately proved decisive.
O'Shea said: "I thought if we kept it going we were definitely going to score because we were so dominant
Then they get that goal and we recover to take control of the game again but we just couldn't score."
Source : PA
Source: PA