They meet again in the RaboDirect PRO12 final on Sunday, as Leinster return to the Royal Dublin Showground buoyed by their latest Heineken Cup success.
But Tandy has urged his side to crash Leinster's homecoming party and seal the PRO12 crown.
"We have done really well to beat them twice this year but that will count for nothing if we don't win on Sunday," said Tandy.
"We need an exceptional performance to step onto that podium.
"Leinster are a fantastic team and to win the Heineken Cup three times takes some doing. You can reel off the names and there is no real weakness in the team.
"There's a realisation within our squad that to go to Leinster and win we have to be at the very top of our game.
"Getting to the final is a serious achievement in itself but this group is not happy with a runners up medal - We want to win and really impose ourselves on the game."
The Ospreys' formidable pack, based around a core of Grand Slam stars, has been a major weapon this season and will again be at the forefront of the Welsh regions assault on the title.
But Tandy insists there is more to his side than the team that edged past Leinster in Dublin two months ago.
"Up front we have a massive core of internationals and Grand Slam winners," added Tandy.
"It's about maturity, you're looking at guys who are coming of age and already had a lot of top level experience. They've been involved in winning environments and that rubs off on others.
"As a team we have evolved since we played them out there. We have an idea of how we can achieve the result we need, we have prepared for that and it's all about us implementing our game and stopping Leinster dictating.
"The scrum has been a real positive for us this season but against Munster it didn't produce any points. Our points came from our defence, making turn overs and capitalising on them.
"On Sunday we have to be clinical and we can't give a side like Leinster cheap possesion.
"We will cough up possesion at times but it's about how we react and we have to be on the money in defence."