|Ospreys (9) 9|
|Pens: Biggar 3|
|Northampton (17) 20|
|Tries: Clark, North Cons: Myler 2 Pens: Myler 2|
Northampton set up a Pool Five decider with Racing Metro next week as they ended Ospreys' Champions Cup hopes with a 20-9 win at Liberty Stadium.
If Saints beat Racing at home they will go through, but even with a loss could still qualify as a runner-up.
Ospreys were made to pay for mistakes, as Calum Clark capitalised on a charge-down and George North picked off a dropped ball to score first-half tries.
A Stephen Myler penalty in the second half sealed victory for Saints.
Ospreys had to settle for three Dan Biggar penalties, all scored before the break, and have just pride to play for in the final round of matches next week against Treviso.
Going into the game, the Welsh side had not lost at Liberty Stadium this season, but the Pro12 leaders have not enjoyed a good record against Northampton in recent history.
Saints, who lead the English Premiership, beat the Welsh region 34-6 on home turf in round two in October, and last season handed Ospreys their heaviest home defeat by an English team with a 29-17 win.
The Ospreys scrum has suffered with injuries this season, but Marc Thomas, Scott Baldwin and Aaron Jarvis were restored to the front row after starting on the bench in the win against Newport Gwent Dragons last week.
Saints had come unstuck at Sale in their last outing, losing 20-7, but had England hooker Dylan Hartley back after suspension, while Christian Day and Samu Manoa also returned to the pack.
Wales international North had scored four tries against his countrymen at Franklin's Gardens in their last meeting and Ospreys showed from the start that they intended to deny the British and Irish Lions wing space to work in.
|European Champions Cup Pool Five|
That concentration in defence helped pen back Northampton and the pressure earned Biggar a penalty kick to give Ospreys an early lead.
But Saints - who lost full-back Ben Foden early to injury - turned the tables when Tom Wood charged down Rhys Webb's attempted clearance and Clark scooped up the loose ball for the game's opening try, converted by Myler.
The Northampton fly-half and opposite number Biggar exchanged penalties, before Biggar was upended by George Pisi in the tackle - earning the Saints centre a yellow card.
Biggar picked himself up and kicked the resulting penalty to bring Ospreys to within a point, before another error gifted Northampton a second try.
Justin Tipuric lost the ball as he was tackled by Luther Burrell and North was on it in a flash, racing away to secure another seven points for the visitors and a 17-9 half-time lead.
Northampton targeted the Ospreys scrum after the restart and the home eight had to operate largely on the back foot.
That physical confrontation and the efforts being made in defence took its toll, but the Welsh side still managed to maintain discipline and limit Northampton to a solitary Myler penalty.
There was enough in the tank for Ospreys to stage an assault on the Northampton line in the final 10 minutes, but forays by Tyler Ardron and replacements Dmitri Arhip and Hanno Dirksen were stopped just short.
Northampton Saints head coach Jim Mallinder said:
"We're delighted. It wasn't the prettiest but we took our opportunities when we had them, defended really well and I'm pleased with the response from last weekend.
"Next weekend will be brilliant. A home quarter-final would be awesome.
"You have to keep going but it's all to play for. Racing are a quality side and I think it'll be a great game."
Ospreys: Dan Evans; Aisea Natoga, Ashley Beck, Josh Matavesi, Eli Walker; Dan Biggar, Rhys Webb; Marc Thomas, Scott Baldwin, Aaron Jarvis, Rynier Bernardo, Alun Wyn Jones (capt), James King, Justin Tipuric, Tyler Ardron.
Replacements: Sam Parry, Gareth Thomas, Dmitri Arhip, Morgan Allen, Sam Lewis, Martin Roberts, Sam Davies, Hanno Dirksen.
Northampton: Ben Foden; Ken Pisi, George Pisi, Luther Burrell, George North; Steve Myler, Lee Dickson; Alex Waller, Dylan Hartley (capt), Gareth Denman, Sam Dickinson, Christian Day, Tom Wood, Calum Clark, Samu Manoa.
Replacements: Mikey Haywood, Ethan Waller, Alex Corbisiero, Phil Dowson, Teimana Harrison, Joel Hodgson, James Wilson, Jamie Elliott.
Referee: Jerome Garces (France)
Touch judges: Patrick Pechambert (France), touch Judge 2 Jonathan Gasnier (France)
Fourth official: Stuart Vaudin (Wales)
TMO Jean-Marie Piraveau (France)
Citing commissioner: Maurizio Vancini (Italy)