Antoine Semenyo, O'Reilly and Erling Haaland struck the goals for City on Wednesday night.
On Bernardo, who was taken off on the hour mark, Guardiola said: “He’s irreplaceable. When Bernardo is not there we have to play another system.
“He presses to the keeper and then helps Rodri build up and then sometimes goes to the right like at Anfield to make crosses or passes to win the game. This is Bernardo.
“(Our performance was) really good. One of the best of the season, (especially) after Anfield for our experience there, in an important, emotional game.
“The next one is always tricky. We were lucky we were home, not away. In our first game there (at Fulham), we suffered incredibly.
“The second half, we didn’t run there. We ran today. We adjusted a few things in our pressing. In the second half they made a step up and were more aggressive.
“Bernardo was exhausted, Rodri was exhausted. He didn’t lose one ball and then in the second half he lost 10 balls. The back four was so solid.
O'Reilly top, top, top
Guardiola also praised O’Reilly's performance.
He said: “Top, top, top. We need physicality there because we are not incredibly composed with the ball and if we are not strong enough we will not compete.
“With Bernardo, Rodri and Nico in those positions we have a base. With Antoine, we have rhythm and pace in our pressing and that helps.
“We have to be close to each other. That is the only way to defend and in the first half that was outstanding and really good.
“I always say he has incredible pace going forward. Erling kept the ball and after that the pace for Nico and Antoine make the difference.
“Now we have Salford then we need to not see each other and rest. We have an incredibly tough schedule like Arsenal, and we will have a midweek off."
“It’s been so tough and with a lot of injuries. Salford and then we need two, three, four or five days off. We will try to prepare the next two games before we arrive in March.”