And, with Priestland absent from the Scarlets line-up at the Liberty Stadium, Biggar further strengthened his cause with an impressive display in a 32-3 win.
The Ospreys had won their last five clashes with Welsh regions heading into the derby encounter and, thanks largely to their set-piece superiority, they didn't look like doing so on Wednesday.
But for some wasteful play in the final quarter the Ospreys could have been celebrating a bonus-point win after Aled Thomas (yellow) and replacements Gareth Owen and Rob McCusker (both red) all fell foul of referee Nigel Owens late on.
In the end the home side had a penalty score plus tries from James King and Kahn Fotuali'i - who also kicked a drop goal - to add to Biggar's haul with the boot.
The Ospreys were ahead as early as the second minute with a Biggar penalty from in front of the posts, after the Scarlets were penalised at scrum time.
It was 6-0 on 11 minutes when Biggar added a drop goal to his tally as Ospreys dominated the early exchanges.
The Ospreys had the advantage at the set piece and they made that pay on 24 minutes when referee Owens awarded them a penalty try.
Biggar again made no mistake with the conversion and Scarlets found themselves 13-0 down.
That margin was reduced two minutes later when a Thomas penalty got the Scarlets up and running however there was more bad news for the West Walians just before the half-hour mark.
Second-row George Earle was sent to the sin-bin to heap further pressure on their pack however it was 3-3 - a Thomas and a Biggar penalty - while the South African was off the field.
Earle re-entered the fray a minute before the interval only for the Ospreys - through a Biggar penalty taking his first-half tally to 11 points - to extend their lead to 19-3.
The home side started where they left off after the break and had a second try when King crossed in the corner.
Kahn Fotuali'i caught the Scarlets napping with a quick tap penalty and when the ball was recycled wide the second row crossed.
Biggar missed the conversion but did nothing of the sort with a penalty on 50 minutes to make it 27-3.
The Ospreys got their third try when Fotuali'i touched down, just, in the corner, after holding off Scarlets fly-half Thomas' tackle, Biggar missing the conversion.
The Scarlets No. 10's petulance then saw him given a yellow-card in the 71st minute as the Ospreys looked for that all-important fourth score.
The visitors ended the game with 12 when McCusker and Owen were both red-carded late on but the Ospreys weren't able to add a fourth try.
|Ospreys Score Card|
|Scarlets Score Card|