Now, with a return to Heineken Cup on the cards, the Ospreys will be looking to move off the bottom of what is a hugely competitive Pool 1. They take on a Saints side who are still chasing qualification and secured an impressive away win over Leinster in the last round.
"We've put together a run of wins in recent weeks, starting with the win over Castres in the last round but we aren't kidding ourselves about where we are at in terms of performance," said Tandy.
"We are improving week on week but to get a result against a team of Northampton's stature, we will need to move up several levels.
"There's been a great mood in the camp this week - the boys are determined to represent the jersey positively and we're looking forward to a big afternoon of rugby. It's important we get a big crowd, creating a special atmosphere. We had a taste of what the Ospreys supporters can do at last week's game against the Scarlets and we need them to do the same again this afternoon."
For the clash with the English Premiership side, the Ospreys have named a powerful looking team. The pack is chock-full of Welsh international experience with Alun Wyn Jones leading the side and Sam Lewis looking to continue his good recent form in the back row. Wales prop Adam Jones settles for a place on the bench.
In the backs, Richard Fussell returns at full back with Sam Davies among the replacements. Rhys Webb partners Dan Biggar at half back.
Tandy added: "Northampton are flying high in the league and Europe and both teams need the win. It's going to be explosive and physical, I'm sure it'll be a great contest.
"These are the kinds of games that players, coaches and supporters want to be involved in. The results in the last round mean there is a chink of daylight for us and you can guarantee we'll going for it. We know that to have any hope of progressing, a win on Sunday is essential.
"We can still make it through the pool stages in Europe if we get the performance right and things fall into place elsewhere. We are in a good position in the PRO12 so it's crucial we remain focused, believe in what we are doing and keep the momentum we've built in recent weeks going."
Saints have named a strong side themselves with England internationals Stephen Myler, Dylan Hartley, Courtney Lawes and Tom Wood all starting. George North is a familiar face on the wing while former Osprey Kahn Fotuali'i goes head-to-head with Webb at scrum half.
On the task of taking on North, Ospreys and Wales centre Ashley Beck said: "We all know the huge threat that George poses and it was interesting seeing him playing in the centre against Leinster.
"If he wants to try to pop up in the midfield at the Liberty then we will be ready for him, our defence has been good for most of this season."