Having beaten the Blues 15-12 in last month’s rearranged game at Beddau, the Scarlets won 18-0 at Llandovery to make it four wins in six games so far this season.
Luke Williams crossed twice for the hosts with Dale Ford adding two penalties and a conversion for good measure.
The Ospreys put up a decent show in the first half, but struggled after that and slipped to their first defeat of the season.
