The two sides may have embraced regionalism but the links to the old Llanelli versus Swansea and Llanelli versus Neath derbies ensure there is little love lost between two of the most successful areas in the history of the Welsh game.
And while the Scarlets may have enjoyed a promising start to the new season, it's the Ospreys who currently hold the bragging rights after home and away successes in last season's derby clashes.
For Rees, it's payback time after last year's December 26th defeat at the Parc-Y-Scarlets left him feeling anything but festive.
"Losing at home to the Ospreys has got to be the worst feeling ever," Rees told magnersleague.com.
"We lost at home to them on Boxing Day last year. I didn't play that game but it was really frustrating watching it and it's important that we turn things around this year.
"You look forward to playing in big games like this. It's so easy to get up for it. It's a local derby and you're playing for a place in the Welsh squad as well.
"It's probably our biggest game playing for the Scarlets against the Ospreys at our home ground. "
While the Ospreys finished last season with Grand Final glory, it's the Scarlets who are flying high this time around.
After putting a shock opening-round defeat to Treviso behind them, Rees and co have since recorded three straight wins and scored 13 tries in the process.
The most-recent of those victories came in another Welsh derby at the Dragons but Rees insists the most pleasing aspect of the new campaign has been the Scarlets' home form.
Having lost four of their eight league games at the Parc-Y-Scarlets last term, Nigel Davies' men are unbeaten there this time around.
Bonus-point wins over Connacht and Aironi have given Rees exactly the kind of start he asked for when he emphasised the importance of home form when giving one of his first team talks as Scarlets skipper.
The Scarlets have started with two home wins in West Wales
"The biggest thing for us is that our home record last year was poor. We only won 50 per cent of our home games. That record is poor and it's something that we need to change," added Rees.
"That's our biggest challenge this year, to make sure that we win our home games and then pinch a few on the road.
"We've got to make our home games a priority. We've played two and won two at home so far and that's the way we need to be.
"It's taken us a while to settle in here at Parc-Y-Scarlets but it's definitely the way forward. We've got a great stadium with great facilities. It was also expected that it would take us a while to settle in but we've got no excuses now. This is our third season here and it's important that we make it a tough place to come.
"For me, as captain of the Scarlets, the main thing is that we get that fortress feeling back when teams come down here.
"We need to make sure that, when we do play at home, we make a statement.
"We want to make a statement and get the Scarlets back up where we believe we belong and where we have been in the past. But it's about getting that foundation in place first."
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