The Liberty Stadium needed to triumph to keep alive any realistic hopes of advancing in Europe. But they were unable to triumph despite an impressive performance from Wales fly-half Dan Biggar who racked up a try and 11-points from the boot.
The defeat sees the Ospreys remain rock bottom in Pool One and all but mathematically out of Europe.
Northampton made a thunderous start to the encounter and earned a penalty inside 30 seconds, which Myler slotted.
They continued to play the game at the ferocious pace and Manoa cantered over after Ryan Jones was caught napping at a dominant Saints scrum.
Myler added the extras and the Dan Biggar knocked over quick-fire penalties as the Ospreys clawed their way back.
But the Saints took a major stride towards victory as their strike runners caused the Ospreys problems once again.
Luther Burrell crossed the gain-line twice and following several phases Phil Dowson and Courtney Lawes displayed slick hands to put Day over in the corner.
Mylar slotted the difficult conversion to hand his side a 17-6 lead at the interval.
But whatever a visibly angered Steve Tandy said in the changing rooms did the trick as the Ospreys responded in the second-half.
They piled the pressure on the English club right from kick-off with both Richard Hibbard and Alun Wyn Jones both coming close.
And the Swansea-based outfit got their reward when Biggar twisted and turned his way over to score, adding the simple conversion.
But they were dealt a major blow when Foden scampered over for Saints’ third try. A powerful driving lineout threatened to breach the Ospreys line and after they held it at bay the ball was released to Day, who released Foden with a sublime quick-pass.
Myler was once again on target to stretch his side’s lead to 11-points. Biggar chipped away but Myler was more than a match and cancelled out his efforts.
Saints pressed for a bonus-point try at the death but they were unable to cross the whitewash despite Biggar being sent to the sin-bin.
