However, Tandy is far from satisfied with a top-four finish, ahead of their concluding matches of the regular season against Glasgow Warriors and Connacht.
And he's quickly reset his sights on securing home advantage in the semi-finals - and then booking a slot in the final showdown in Belfast.
"We are happy and we enjoyed the moment but our season has a long way to go and we want to do better," said Tandy.
"No one wants to exit this competition in the semi-finals, we want to be in Belfast and challenging for the title.
"There are things to improve but there is a sense that we are building something at Ospreys. We've got youth but then you add in experience like Alun Wyn Jones and special things can happen."
Dan Biggar was the star of their Judgement Day victory with a 16-point contribution but it was captain Jones that caught Tandy's eye.
"He's a sensational player, a world-class international," he added. "He's a special player and a special guy and a very special captain."
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