The result means Scarlets will have to settle for playing, for the first time, in the second-tier competition – the Challenge Cup.
Ospreys’ victory came through tries for George North, Olly Cracknell and replacement wing Hanno Dirksen, who was also yellow-carded in the first half. Sam Davies kicked two conversions and Luke Price one.
The play-off came about with Ospreys and Scarlets finishing fourth in Conference A and B respectively, with no Welsh team qualifying automatically for the 2019-20 Champions Cup.
Ospreys had the better of the first half to lead 14-10 at the break but the Dirksen yellow card had brought Scarlets right back into the contest.
But the home side’s attacking intent and an efficient line-out produced the opening try on nine minutes when Watkin put North over on the right. Davies converted.
Halfpenny did get Scarlets on the scoreboard with a penalty five minutes late.
However, a loose kick by wing Ioan Nicholas from deep inside his 22 allowed Ospreys to double their
score. Dan Evans counter-attacked and Cracknell was put in under the posts.
Davies converted as Ospreys led 14-3 on 18 minutes.
But the yellow card for Dirksen for a dangerous challenge in the air against McNicholl suddenly changed the momentum.
A minute after the sin bin Jonathan Davies went over after a dart down the right by McNicholl. Halfpenny converted to reduce the deficit to four points.
At the start of the second half Sam Davies missed a 45-metre penalty and then scuffed a drop-goal attempt.
The turning point in the second half came when replacement fly-half Rhys Patchell missed touch with a penalty attempt.
By contrast Price found touch with his kick a minute later and from that position Ospreys scored their third try when Price put Dirksen over on 71 minutes. Price converted to all but seal the region’s place at Europe’s top table.
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