Gus Poyet's Sunderland looked dead and buried, but beat the drop after a run of four consecutive victories that included wins against Chelsea and Manchester United.
Sunderland's safety was officially confirmed on Wednesday with a 2-0 triumph at home to West Brom - a result that moved the club five points above the relegation zone with one match remaining.
And Irishman O'Shea, who won five Premier League titles and a UEFA Champions League crown amongst other trophies during his 12-year-spell with Manchester United, believes preserving the club's top-flight status has gone close to eclipsing some of his biggest achievements.
"It's very close to the top of what I've done because of the points we were down and the games we still had to play," said the 33-year-old defender.
"But we've done it, and we've done it stylishly as well.
"It would have to rank up there amongst it all because we were dead and buried, and everybody had written us off. But we've come good with great football, great results, and done it with a game to spare."
O'Shea added: "Congratulations is a strange word because it's (relegation) something we feel we shouldn't be near.
"If you're talking about the squad that we have, the belief and the quality, we shouldn't be down there."
Uruguayan Poyet played a key role in guiding Sunderland to survival, having replaced Paolo Di Canio earlier in the season.
O'Shea wasted little time paying tribute to Poyet, who has recently been installed as favourite to take over from Sam Allardyce at Premier League rivals West Ham next season.
"After seven or eight games, to have one point, we were looking up the table for 99 per cent of the season," he said.
"It was difficult, but credit to the manager, the staff and the squad because we stuck to the ideas the manager was trying to get across to us."
