Madrid, who lost 3-2 here to Milan at the same stage of last season’s competition, could hardly have asked for a better start as they swept into a two-goal lead.
The opening goal came after Madrid won a free-kick on the edge of the box, Xabi Alonso being pushed over by Alexandre Pato.
That gave Ronaldo a chance to net his fifth goal in three games following braces against Deportivo La Coruna and Malaga, and the Portugal international did not waste the opening, firing his shot at waist height through a poor Milan wall to leave visiting goalkeeper Marco Amelia with no chance.
Milan coach Massimiliano Allegri, who sent out an adventurous line-up, will have been far from happy with that goal, but worse was to follow for the Italians 60 seconds later as Madrid doubled their lead.
Ronaldo was again involved, setting up Ozil inside the area and the German’s right-footed shot took a huge deflection off Daniele Bonera before looping into the net with the already-committed Amelia again helpless.
Madrid looked in complete control but Milan came within a whisker of pulling a goal back in the 28th minute when Andrea Pirlo fired a 20-yard free-kick against the crossbar.
Six minutes after that Milan had another chance but former Madrid player Clarence Seedorf, having been put in by Ronaldinho, blazed over from 12 yards out.
The remainder of the half was all Madrid, and Amelia did well to keep out efforts from the fired-up Ronaldo and Ozil, while Angel Di Maria twice threatened as Mourinho’s men looked keen to kill the game off before the break.
Madrid continued where they left off after the re-start and created two good openings in the first 10 minutes, but neither Gonzalo Higuain or Di Maria could make the most of them.
With Milan struggling to break down a Madrid defence that had only conceded three goals in nine games under Mourinho this season, the hosts were the only side looking likely to add to the scoreline.
Higuain, Ronaldo and Pepe might all have scored before Allegri introduced Robinho in the 71st minute in a bid to change the flow of the game.
Robinho, who left Madrid in acrimonious circumstances in 2008, was welcomed onto the pitch by a cascade of jeers from the Bernabeu crowd and the Brazilian received similar every time he touched the ball.
Robinho did at least give Madrid goalkeeper Iker Casillas something to do, drawing a good diving save out of the Spain number one in the 83rd minute, but that was as good as it got for Milan as they slumped to a deserved defeat.