Northampton pushed hard in a second-half fightback but couldn't overcome Ospreys as they fell to a 43-32 Champions Cup defeat at Franklin's Gardens.
Jeff Hassler and Dan Evans both scored twice for Ospreys, with Tom Habberfield and Kieron Fonotia also dotting down and Sam Davies adding five conversions and a penalty in an accomplished fly-half display.
Northampton did produce some late fireworks, scoring five times in the second half through skipper Dylan Hartley, Nic Groom, Rob Horne, Ahsee Tuala and Harry Mallinder, but it was not enough to prevent a ninth defeat in 10 games in all competitions.
Mallinder and Ospreys' Rob McCusker were sin-binned for their part in a brawl late on and the visitors' Hanno Dirksen saw red for a high tackle on JP Estelles.
Northampton have now been beaten in all three Champions Cup matches and their hopes of progression from the pool stages are already over.
Stephen Myler, brought on early after Piers Francis suffered a head injury, initially put Saints ahead with a simple kick from in front of the posts.
Ospreys wing Hassler was yellow carded during the opening exchanges but the away side immediately levelled the scores via a Davies penalty.
Northampton then lost a second fly-half to injury, with Myler forced off and Mallinder moving from full-back to 10, and things got worse when Ospreys put together a tidy move that was finished by full-back Evans.
Ospreys were to score again before the break, with an even more impressive attack resulting in a try for Habberfield.
The Welsh outfit didn't have to wait long for their third score after the break as Hassler was set free down the left on 46 minutes.
Davies kicked the conversion and he was doing so again soon after as Fonotia grabbed his team's bonus-point score.
Hartley finally gave the Saints supporters something to shout about, but Mallinder missed the conversion and Evans quickly scored Ospreys' fifth try.
Davies converted again to make it 38-8 and there was seemingly no end to the torture as Hassler completed his hat-trick.
Scrum-half Groom got a score back for Northampton, Horne got on the scoresheet and the Saints had their bonus point when Tuala finished off a move that included a fine offload from Mallinder.
Tuala then returned the favour for the Saints' fifth score, offloading to Mallinder, who raced from halfway to score a superb try.
Mallinder and Ospreys replacement McCusker were sin-binned for their part in a brawl that followed the try, meaning Northampton had to hand the conversion to scrum-half Cobus Reinach, who hit the post.
Ospreys were reduced to 13 men before the end as Dirksen was red carded for a horrible high tackle on Estelles, but the Welsh side saw the game out to deny Northampton a losing bonus point.
