Unfortunately for Leinster and Ireland, it subsequently emerged that O'Brien suffered a fractured leg and will be out of action for the immediate future.
The Scarlets will be kicking themselves as this was a match they could have easily won. They failed to convert a number of gilt-edged try-scoring chances, the most notable of which saw Rob McCusker knock on after blocking down Shaun Berne's 70th-minute clearance kick.
Jonathan Davies and the fit-again Regan King showed well early on for the visitors but when Deacon Manu was isolated at a midfield ruck on 11 minutes, Sexton smacked over the resulting penalty for the opening score.
Davies rumbled forward to set up Rhys Priestland for a levelling penalty and the Scarlets surged 8-3 ahead, just a minute later, courtesy of one of the best tries of the season.
A fantastic long range break, highlighted by deft passes from King and Johnathan Edwards, caught Leinster napping and hooker Ken Owens, chasing his first Welsh cap against France next weekend, finished it off in the left corner.
Priestland failed to add the extras and Leinster soon increased their intensity, big prop CJ van de Linde was held up short before space was created on the left for Nacewa to go over.
Sexton nailed a terrific conversion from close to the touchline and swapped further penalties with Priestland as Leinster went in at the break, 13-11 up.
When Stan Wright was pinged for not rolling away, four minutes into the second half, Scarlets out-half Priestland kicked his side back in front but ultimately, luck was not with the Welsh region.
Daniel Evans was inches away for sending King over for a try - Fergus McFadden doing well to intercept close to the Leinster try-line - and the Scarlets pack was thwarted at close range on a couple of occasions as Shane Jennings and his team-mates kept them at bay.
Three well-struck penalties from the in-form Sexton, after 48, 57 and 67 minutes, stretched Leinster more than a converted score clear and he ran a good support line, following a line break from Berne, to take a pass from debutant Andrew Conway and score the clinching try, four minutes from time.
Sexton pushed his conversion attempt to the right of the posts and salt was rubbed into the Scarlets' wounds when Ireland Under-20 starlet Conway recovered well to force a knock-on from Andy Fenby, just metres from the home line.
|Leinster Rugby Score Card|
|Scarlets Score Card|