Ulster fell to their fourth defeat in five Magners League games on a miserable night at Ravenhill which saw the Ospreys win 16-13, their fourth league victory in a row enough to keep them third in the table.
Simon Danielli's 68th-minute try had given Ulster the lead but a 75th-minute touchdown from Ospreys number eight Filo Tiatia, which was converted by James Hook, saw the visitors shade the win.
With former Ulster player Tommy Bowe looking on as a water carrier from the Ospreys bench on his first return to Ravenhill since moving to Wales, the severely depleted visitors had the best of the early exchanges with two James Hook penalties putting them 6-0 ahead after the first quarter.
Even though Ulster's scrum looked to clearly have an edge on their opponents, there was little of note from the home side with the ball in hand or through their tactical kicking, apart from several Danielli bursts out of defence and a solitary Clinton Schifcofske penalty just after the half-hour mark.
The rest was mostly pretty grim. Hook's inventive grubber kick almost created a score but Danielli's timely intervention stopped Tiatia from putting Steve Tandy in some space.
Ospreys also nearly broke through after Gareth Owen's burst past Robbie Diack saw Ospreys held up in the left corner. But that, along with missed drop goals from Hook and Niall O'Connor, was largely all the first-half action.
The second half opened in a similar style, with some drama entering proceedings after an Isaac Boss clearance was charged down by Tom Smith, only for the Ospreys to fail in pouncing on the loose ball bobbling close to Ulster's line.
Ulster then had a rare moment of menace when Paddy Wallace's hack through saw Boss wrap up David Bishop, forcing the Ospreys winger to concede the penalty. However, Schifcofske under-cooked the kickable attempt.
The Ulster full-back had another penalty chance from the same range after 62 minutes but this time pushed it wide. Three minutes later he finally made sure from a more favourable angle to level the scores.
Then O'Connor launched a cross kick which sat up nicely for Danielli and the Scotland winger was clear, spectacularly diving under the posts to give Schifcofske the straightforward conversion to put Ulster into the lead for the first time in the 68th minute.
The lead was cut by Hook when Ulster were penalised from the restart and, from there to the end, the Ospreys had the upper hand.
Danielli just got to the ball to prevent a try when Jonny Vaughton's chip-and-chase nearly saw the winger get the touchdown.
But Ulster were pegged back in their own 22 and after a series of pick and drives, Tiatia burrowed over and was given the score by the television match official.
Hook converted and Ulster could not threaten again over the last five minutes.
The RBS 6 Nations Championship 2015 was not poised to go down as one of the more "vintage" Championships, though there was great anticipation as the action headed into its final day. What transpired was something that nobody could foresee: "The Greatest Day in Rugby's Greatest Championship."