With Castres now out of contention for qualification, the Dragons know that victory with a bonus point against the Falcons will clinch the last place as one of the best runners-up. As well as helping a fellow Welsh region, the Ospreys were desperate to end their campaign on a high.
Preparations had been hit by a number of injuries but it took only seven minutes for new scrum-half Richard Rees to give them the lead after chasing Gavin Henson's kick. Henson converted and then traded penalties with Laurent Marticorena before Castres flexed their muscles to take the lead through France prop Alessio Galasso.
The Ospreys were forced to dig deep when Rees was shown the yellow card and again had their backs to the wall after Henson had snatched back the lead with a penalty. Castres threw caution to the wind with qualification still at stake only for Seveali'i to strike in injury time.