The Scarlets overcame an Ospreys fightback at St Helen's to move back to the top of the Celtic League.
Ospreys (3) 15
Tries: Terblanche, S Williams
Scarlets (15) 18
Tries: Thiel, Bowen
Pens: S Jones (2)
Tries from Jon Thiel and Barry Davies gave them a 15-point second-half lead.
A Stefan Terblanche interception score and a Shane Williams try reduced the gap to three, but Gavin Henson missed the game's last kick, a 44m penalty.
The Ospreys gained a bonus point, leaving them four ahead of the Blues in the battle for the final Welsh Heineken Cup place.
They travel to Glasgow next week, whilst the Cardiff side welcome Edinburgh.
In the race for the title, the Scarlets lie one point ahead of the Dragons and Ulster, who travel to Stradey Park on Friday.
The Dragons face a testing trip to Leinster.
A furious opening assault by the Scarlets was rewarded when Barry Davies broke inside the Ospreys 22, showing the strength to
hold off Terblanche before feeding Thiel for a five metre run-in.
Chris Wyatt went close to a second try, but the ball was dislodged from his grasp by a Williams tackle as he crossed the
Stephen Jones had missed with a relatively easy conversion and penalty, but after 18 minutes he was able to extend the
Llanelli side's lead to 8-0.
The home side gradually worked their way into the game, and on 32 minutes they saw the Scarlets reduced to 14 men when Scott
Quinnell was yellow carded for punching Huw Bennett.
The Ospreys took advantage to drive down to the visitors' line, and when Dave Hodges infringed at a ruck a Henson penalty
brought it back to 8-3.
But with the final move of the first half replacement fly-half Bowen broke clear of Andy Newman in midfield, toyed with a
back-tracking, off-balance Williams and crossed under the posts.
Jones, back on for Bowen after having his head bandaged, opened the scoring in the second half with a penalty.
The game was drifting from the Ospreys as the Scarlets pushed for a third try deep in their 22.
But a long Quinnell pass in midfield was intercepted by Terblanche who sprinted home from 80 metres.
Scarlets hooker Robin McBryde was then sin binned for hands at a ruck, and the home side fired back into their half.
A chip through from Henson bounced fortunately for Dave Tiueti, the centre feeding Williams who crossed in the corner to
reduce the gap to three points.
Gareth Bowen missed with a long-range penalty in the 78th minute, before Henson got a final chance to snatch a draw.
The kick easily had the length but drifted agonisingly to the right of the posts.
"It was disappointing to miss, I really thought I'd got it over," said Henson.
"It was good we made a game of it after a terrible first half, but now we just have to make sure we go up to Glasgow and win."
Scarlets director of rugby Gareth Jenkins was delighted that his side's title ambitions were still in their own hands.
"That was far too close for comfort, it would have been a disaster if that last kick had gone over," said Jenkins.
"It was disappointing that it came down to that as I felt we had done enough to win by a comfortable margin."
Ospreys: G Henson, S Terblanche, J Storey, D Tiueti, S Williams, S Connor, A Williams, D Jones, B Williams, A Millward, A Newman, J Thomas, J Bater, S Tandy, A Lloyd.
Replacements: R Wells, A Durston, E Seveali'i, H Bennett, P James, N Bonner-Evans, R Pugh.
Scarlets: B Davies, G Evans, M Taylor, M Watkins, S Finau, S Jones, D Peel, I Thomas, R McBryde, J Thiel, V Cooper (capt), C Wyatt, D Hodges, S Easterby, S Quinnell.
Replacements: J Davies, A Gravelle, B Griffiths, D Jones, M Phillips, G Bowen, L Davies.
Referee: Gregg Davies (Scotland)