Ospreys edged a dramatic and pulsating Welsh Pro12 derby as Scarlets fly-half Steven Shingler missed a last-minute penalty.
The visitors have now won six of their seven west Wales Boxing Day clashes against the Llanelli-based team.
John Barclay and Aled Davies went over for the home side's tries with Hanno Dirksen and Alun Wyn Jones replying before the break.
Dan Biggar converted Eli Walker's try to give the visitors victory.
The home side's roller-coaster season had put them at the top of the Pro12 with eight wins from nine starts and bottom of Pool 3 in the European Champions Cup with four defeats.
That was in clear contrast to Ospreys' record of only four previous wins in the Pro12 while sitting top of Pool 2 in Europe's top-tier tournament.