Ton fell to the Terrors in the second half, despite battering United in the first half of the first-leg between the two sides.
Oliver believes it’s important that the Greenock men get back to their free-flowing ways in order to cause problems at Tannadice.
He told the Tele: “We’ve got some great footballers in the team. We just need to be confident to get on the ball in these games, we need to want to get on the ball. You can’t just clear your lines because we’re not the biggest, most physical team in the league but we have good football players.
“On the ground we’ll give most teams problems.
“In the second half of Tuesday’s game we were just clearing our lines a bit but we need to try and pass the ball better. I’m sure it’s going to be a big 90 minutes in Dundee, it’s far from over yet.
“We are still in the tie. We were back to our usual selves from the middle of the season in a good first half but we went in at half-time and we thought we’d done well on Tuesday.
“We had, but we wanted to keep it up in the second half but I think they changed formation in the second half, put Simon Murray up front and we didn’t adapt to it well.
“We started the second half too deep, but like we’ve said, it’s only half-time in the tie and there’s another 90 minutes to play. We only need one goal so we’ll try our best to get it.
“I don’t think we adapted to their change as well as we’d have wanted, we obviously knew how they played before the game but after half-time we didn’t adapt.
“It just worked in their favour but our boys maybe needed to identify what was happening but we didn’t.”
Of his own performance on Tuesday evening, Oliver said: “I felt back to my usual self, my touch was good, I felt sharp, it’s the best I’ve felt in a few weeks actually. I felt back to my usual fitness and sharpness. The gaffer gave me a few days off just to rest my legs and it did me the world of good.”
The striker is delighted with the support both he and his team-mates have received from the Cappielow faithful and he's hopeful that a significant crowd travel to Tayside to see it through.
Oliver he believes, regardless of tonight's outcome, fans should be as delighted as the team with Morton's overall performance this season in the Ladbrokes Championship.
He added: "The fans have been brilliant this season and it obviously helps when you hear your supporters singing, it gives you a wee lift so hopefully they can bring the numbers there and it'll be good for the club. We hope it's not our last game of the season but it'll give us the best chance of getting a result.
"They should be proud of the season the club has had. We're not necessarily a big, massive team, and the boys have been fantastic. At the start of the season I'm sure the fans would've bit the hands off you to be where we are now and we've got to remember that."
Meanwhile, goalkeeper Jamie McGowan and young striker Ben Armour have signed contract extensions to extend their stays with Morton.