The Cobblers produced their worst defensive performance of the season to slump to an embarrassing home defeat to a rampant Accrington.
Marc Richards maintained his goalscoring streak and Ivan Toney registered a double off the bench but it was not enough as Town were submerged by five Stanley goals in a 5-4 loss.
Town made a single change from the team which started against Hartlepool, with Lee Collins replacing the injured Zander Diamond in central defence.
Richards gave an illustration of his growing confidence with a snap shot from a difficult angle and with a bouncing ball after only five minutes.
Chris Hackett won the game’s first corner moments later with a trademark cross as the home side attempted to get quickly into their stride.
But, as on Tuesday night, it was the away side who broke the deadlock.
The defence failed to deal with both a corner and the follow-up cross and when the ball broke to the left wing, Kal Naismith had time to pick out a cross which was met on the dive by the head of Atkinson.
Five minutes later the Cobblers briefly restored parity, with Richards continuing his excellent goalscoring streak by heading home Joel Byrom’s corner at the back post.
But within a matter of seconds, the visitors had retaken the lead, and from an attacking corner for Northampton.
The ball was hooked clear and Town’s two covering players, Gregor Robertson and Danny Alfei, both went for it and missed it, and O’Sullivan was presented with a clear run at goal.
He did well to dummy Archer and then rolled the ball into the empty net.
Archer knew little about Accrington’s third but it will go down as an own goal for him, Atkinson’s header hitting the post and rebounding in off the goalkeeper’s leg.
The introduction of Ivan Toney 10 minutes into the second half gave Northampton a little more in attack but they still struggled to create much of note.
And it was Accrington who scored the next goal of the game, O’Sullivan finishing in style after an excellent solo run through some static defenders.
Toney gave the supporters something to cheer when he poked in from close range after a Gregor Robertson but the glory was fleeting.
The final insult was provided by Sean McGuire, who produced a subtle finish after nipping in between the central defenders.
Toney got his second with a firm header from Kaid Mohamed’s left-wing cross and Darren Carter smashed in a loose ball to cut the deficit to one in injury time.
But it all came too late in the game for anything to be salvaged.
Cobblers: Archer; Alfei, Cresswell, Collins, Robertson; Watson, Byrom; Hackett, D’Ath, Mohamed; Richards
Subs: Toney (for D’Ath 56), Carter (for Robertson 78), Tozer (for Alfei 78)
Not used: O’Toole, Snedker, Ravenhill, Moyo
Accrington: Lumley; Hunt, Aldred, Winnard, Atkinson; O’Sullivan, Joyce, Procter, Naismith; McGuire, Carver
Subs: Windass (for Carver 70), Hatfield (for O’Sullivan 82), McCartan (for McGuire 90+1)
Not used: Buxton, Simpson, Mingoia, Bowerman
Away fans: 68