Liverpool were spanked black and blue by a determined foe led by the son of a Hampshire doctor bought for nothing who stretches the tape a full 6ft 6in.
Matt Smith, a free transfer from Solihull Moors 18 months ago, struck within three minutes and again as the tea was brewing ahead of half-time, twin shocks that utterly knocked Liverpool out of their well-upholstered stride. Though Brendan Rodgers' side rallied to within an inch of parity when Steven Gerrard rattled the bar on 90 minutes, there was something in the frigid moorland air that told of a scalping at the end of a momentous fourth round.
Twice in the not too distant past, 1990 and 1994, Oldham had travelled to the semi-final stage in this rumbustious competition, falling each time to Manchester United. In those days Oldham were a first-tier team under the leadership of Joe Royle.
The reality today is somewhat different. Mired near the foot of League One on a run of one point in eight games, the hosts were billed as cannon fodder. Yet, after the stirrings at Brentford and Leeds earlier in the day, Liverpool might have known what kind of welcome was coming.
They would not, however, have reckoned on falling a goal behind so early. A cross from Youssouf M'Changama was met by Smith, a human expression of the Empire State Building, who crashed his header past Brad Jones. The goal sparked a torrid 15 minutes for the visitors, during which Oldham unleashed the warrior spirit of the underdog.
The tackles flew in from both sides, costing Oldham the premature loss of M'Changama in the 11th minute. This was not the defiance of a team on its uppers but one fully engaged in pulling the gentrified tail of an institution that has won the old pot seven times.
And to think the annual flirtation with relegation has had locals sharpening the guillotine for Oldham's manager, Paul Dickov, with some suggesting he would not survive the day win, lose or draw. That was before they pulled off the unimaginable.
There was a sense of crisis off the pitch, too, centred on the matchday pie supply. The club's catering manager could offer a maximum of only 2,500, acceptable when entertaining in League One, but not when Liverpool are in town on the occasion of the fourth round of the FA Cup. A capacity crowd of 10,295 crammed into the three operational sides of Boundary Park, 75 per cent of whom were condemned to disappointment.
More than six inches of snow fell in the early hours of Saturday, every flake of which was cleared later the same day by a platoon of volunteers.
Their reward was a playing surface as knobbly as a horse's knee or as close to perfect as a Lancastrian lawn has any right to be in January. The corner flags were bent double but there was nothing the helpers could do about the cold breath of nature whipping down Sheepfoot Lane.
It took Liverpool 17 minutes to calm the storm, and it was Luis Suarez, the skipper for the day, who stroked his team level. There was a degree of good fortune attached to the build-up, which included a rebound off the knee of Oldham's centre-half, Cliff Byrne, but Suarez was as unerring as ever when presented with a goalkeeper to beat. Dean Bouzanis even did Suarez the honour of diving without convincing anyone that he had an earthly of stopping the shot.
Ten minutes later Liverpool had the ball in the net again when Jordan Henderson's free-kick was deflected home by Suarez. In his eagerness, Suarez had drifted into an offside position and the goal was ruled out.
Liverpool continued to probe, with Suarez, Daniel Sturridge and Raheem Sterling all to the fore. Having dampened local ardour Liverpool were beginning to coast towards the break, a classic error in this environment. Jones was forced into a finger-tip save to deny Smith as the half closed and then, in added time, Smith bulldozed the ball over the line for his second after Reece Wabara had drawn Jones into a fumble.
Liverpool should have been level with their first attack of the second half, Jack Robinson escaping down the left to release Fabio Borini, who scooped his shot over the bar when the goal was far easier to locate.
How the visitors paid for that mistake. When Wabara converted Carl Winchester's speculative cross with a looping header over Jones, a sense of disbelief enveloped the ground, Oldham supporters just as flummoxed as Liverpool's by the turn of events.
Rodgers sought to retrieve his evening with the introduction of Gerrard and Stewart Downing in the 56th minute. Liverpool piled forward with greater urgency, the effort orchestrated through the willing feet of Gerrard. They encountered not only the resolve of Oldham's players but the weather, which spilled across the ground in brutal squalls.
With 11 minutes remaining, Joe Allen's volley deflected off Jose Baxter to give Liverpool hope. And then Gerrard clobbered the bar. Cue delirium and a visit from Everton in the next round.