Victory in that game - all four previous Play-Off semi-finals have been won by the home side - and it could mean a shot at a fourth PRO12 title before Williams brings down the curtain on one of the great careers.
"Getting to the play-offs was the target when we took over so I'm pleased that we've managed to achieve that with a game to spare. It's a real boost for the business and I think there's a real positive feel about the place at the moment," said Tandy.
"Securing the home semi with a game to spare is really important. It means we've got extra preparation time for the game.
"We don't know who we are playing yet, but it's given us another two weeks to prep and get the team ready to go. The uncertainty has been lifted and when we are planning for the next couple of weeks we know exactly where we are and what we are doing."
The Ospreys complete their regular season assignments in Italy on Saturday night when they tackle bottom of the table Aironi. The fact it looks like being that club's final fixture, before the franchise is probably taken over by the Italian Federation, will only make the task more difficult
But that home win over the Dragons made it four wins in a row and boosted confidence, especially as a season high crowd of more than 14,000 turned out to give Shane a cheer in one of his final appearances for the Ospreys.
"It was superb seeing so many supporters there at the Liberty last time out. It really lifted the team and it helped us get the result in the end," said Tandy.
"It was a great occasion and the ending was very fitting for Shane. But you can't lose sight of how important that try was for the team - it wasn't just about Shane, we needed a fourth try for the bonus point to guarantee we have a home semi-final.
"It was a huge moment that has given us that extra bit of breathing space. That could be all important if we are going to be successful in the Play-Offs.
"To have 14,500 in the stadium was amazing, but if we could go even better than that for the semi-final would be fantastic. It would give the boys such a boost to see so many people supporting them in a knock-out game against either Munster or Glasgow."