If West Ham do manage to achieve their golden aim this season and win promotion back to the Premiership, Alan Pardew will probably look back on this amazing victory as a key watershed in his Upton Park career.
Under pressure following Tuesday night's dismal defeat at the hands of lowly Stoke City, the Hammers boss was facing a crisis just weeks into his tenure as Sunderland romped into a two-goal lead after 30 minutes and threatened to condemn his side to their third defeat in the last four home matches.
However, no doubt fired up by a half-time rocket from their boss, the hosts came out for the second half an entirely different team and produced a memorable comeback thanks to Jermain Defoe's double strike and Ian Pearce's late winner.
Defoe's return after his five-game suspension had been a key boost prior to the game but Pardew also made other changes to the team that went down against Stoke. Kevin Horlock came into midfield in place of the injured Michael Carrick, while Robbie Stockdale dropped to the subs bench as the Hammers boss reverted to a 4-4-2 formation that saw Matthew Etherington return from injury to take his place on the left of midfield.
Hammers might have taken the lead in the first minute when Defoe's dangerous free-kick was met by Horlock just six yards out, but the midfielder directed his header just inches wide of the post.
Just two minutes later, Sunderland took the lead, as Pearce's weak back-header was misjudged by David James and Marcus Stewart nipped in to round the Hammers goalkeeper and cut the ball back for Jason McAteer to stroke home.
You could almost sense the Upton Park atmosphere deflate as another vocal and expectant 30,000-plus crowd were stunned into silence - desperately hoping that a repeat of the midweek nightmare wasn't on the cards.
Defoe almost equalised with a shot on the turn in the 22nd minute that cannoned back off the post but, at the other end, Sunderland continued to create danger and things went from bad to worse on the half hour mark as the visitors doubled their lead.
Stewart again broke free and, although his shot was blocked by James, the ensuing challenge for the loose ball saw it loop over to Oster, who gratefully slammed it into the roof of the net.
Frustration understandably began to turn to anger as the crowd voiced their disapproval, while Tomas Repka was booked in the 37th minute for a scuffle with Oster and then exchanged words with his captain Christian Dailly as tempers threatened to boil over.
Pardew made an instant change at half-time, bringing on Don Hutchison in place of Wayne Quinn to allow Marlon Harewood to push further forward but it was the visitors who made the better start to the second half and came just a whisker from making it 3-0 in the 49th minute when Whitley's fierce drive crashed back off the crossbar.
Pardew made another substitution in the 54th minute, replacing the ineffective Harewood with Brian Deane and the change had an immediate effect. A long ball from Dailly was nodded down by the veteran striker and, after Connolly's first effort was blocked by goalkeeper Mart Poom, Defoe pounced to score.
Sunderland boss Mick McCarthy then strangely replaced defensive midfielder McAteer with striker Kevin Kyle, allowing the resurgent Hammers more space deep in the Sunderland half and just five minutes later they were level.
Etherington - showing superb energy in his first appearance for over a month - skipped down the left and cut the ball back for Defoe, who took one touch before firing a left-foot snap-shot past Poom for his 13th goal of the season.
The livewire striker almost claimed a hat-trick in the 78th minute when he latched on to Connolly's pass and fired goalwards, but Poom turned the effort round the post.
Two minutes later, though, the comeback was complete, as Horlock's inswinging free-kick from the right cleared everyone in the penalty area and bounced back off the far post before landing at the feet of Pearce - who gleefully prodded into an empty net from six yards out.
In among the frantic finish, 16-year-old youth prospect Chris Cohen came on for his first team debut - and had Hammers fans reminiscing about Julian Dicks as the young left-back was shown a yellow card for a crunching tackle on the halfway line just moments after his introduction.
The final whistle brought sheer relief for Pardew and his team - while the result proved just how tight things are in a first division table that now makes much happier reading for Hammers as the busy Christmas period approaches. Defeat against Sunderland would have pushed us ever closer to mid-table obscurity but the three points mean that we will travel to Walsall next week in sixth place, eight points off leaders West Brom, who were held at home by Crewe today.