Thousands of fans have already arrived in Cardiff, all desperate to see a repeat of their heroes' gripping 2006 triumph against Biarritz.
O'Connell said: "They are our 16th man.
"I suppose it is something that started when Munster went on a long unbeaten run during the 1999-2000 season.
"You have to give them something to shout about, but that 16th man has really helped us over the line on a fair number of occasions. I hope they can do the same thing tomorrow."
Munster boss Declan Kidney has described today's game against the French giants as the ultimate challenge for his team.
Kidney will head into the Test arena later this summer as Ireland's new coach.
The Munster dream is to give him a memorable farewell as they target a second European title under his direction in three seasons.
But it must be achieved against the most successful club in European Cup history, who have already landed the trophy on three previous occasions.
Kidney said: "The feelings of excitement are palpable.
"We are under no illusions about tomorrow. Everyone really models themselves on Toulouse and the passion and the skill they play with.
"They present the ultimate challenge, but it has been like that for us ever since the pool draw was made 11 months ago."
Munster came through easily the toughest of this season's qualifying pools, topping a group that also included Wasps, Clermont Auvergne and Llanelli Scarlets.
Priceless losing bonus points collected away to Wasps and Clermont ultimately squeezed them into the knockout rounds where they overcame Premiership challengers Gloucester and Saracens.
Kidney added: "We have to roll up our sleeves and go to work again.
"It doesn't get any easier but it is an absolute privilege to be involved."
Kidney has retained the side that edged past semi-final opponents Saracens 18-16 at Coventry's Ricoh Arena four weeks ago.
That means no starting place for scrum half Peter Stringer, a try scorer against Biarritz, who misses out behind Tomas O'Leary, while Munster again unleash a considerable southern hemisphere presence through Doug Howlett, Rua Tipoki and Lifeimi Mafi.
Toulouse show two changes from their semi-final victory over London Irish at Twickenham, with Maleli Kunavore lining up alongside Yannick Jauzion in midfield and France international flanker Thierry Dusautoir replacing Yannick Nyanga.
On the bench, half back Valentin Courrent faces a late fitness test to gauge his recovery from a thigh strain, which means Gregory Lamboley is on standby.
Toulouse, European champions in 1996, 2003 and 2005, have overcome repeated injury problems to reach another final - a fact not lost on their head coach Guy Noves.
He said: "It is the hardest season Toulouse has ever had.
"But despite the injuries, the team has worked together really well and shown you can perform on two fronts. We are in the Heineken Cup final and the French Championship semi-finals."