O'Neill picked spot-kick specialist Guzan over regular custodian Brad Friedel as he made three changes to the side which drew with Wolves on Saturday.
Emile Heskey and Nigel Reo-Coker were recalled at the expense of John Carew and Steve Sidwell as the Villa boss shuffled his pack slightly.
The key word there was slightly as O'Neill realised the importance of the Carling Cup to his fans and picked a compact team which worked well as a unit.
With so few changes, the Claret and Blue held up their hosts at the Stadium of Light and they were grateful to Guzan for saving a Kenwyne Jones spot-kick in normal time.
The big American looked simply unbeatable as he kept out Kieron Richardson when through on goal in extra-time and then saved three penalties in the shoot-out.
Credit must also go to Carew, James Collins and finally Ashley Young who completed the triumph by scoring from 12 yards.
Though it was the players who did the business on the pitch, it was their manager who proved he actually is a selection superman. The decision to pick Guzan was nothing short of inspired and his faith was more than repaid.
Heroics
O'Neill has often been blasted for taking the Carling Cup too lightly and that has been fair comment over the past two seasons with Villa eliminated at the third round stage on both occasions.
But this time round it seems there is a different emphasis at Villa Park. O'Neill wants to do the business in this competition and he now has the necessary tools as his disposal.
The squad is big enough to cope with a couple more fixtures added to the schedule and a trip to Wembley is a possibility provided Villa are handed a couple of favourable draws.
The fans would certainly relish a day out for the first time since 2000 when the Claret and Blue lost to Chelsea in the F.A. Cup final.
And O'Neill finally seems to be listening to his adoring public. Guzan is doing his best to help his gaffer win some silverware with his Sunderland penalty heroics following on from his fine efforts in the Peace Cup.
The super stopper cut and imposing figure in the Malaga showpiece and his spot-kick saving saw Villa claim the trophy with a win against Juventus in the final.
Now he will be hoping to star in every round of the Carling variety of cup competition as O'Neill and his troops march on.