Mark McCrea raced through from far out for Ulster's third try, just short of the half-hour mark. O'Connor's successful conversion made it 17-0.
The closing minutes of the first half saw a far superior Ulster outfit claim their fourth touchdown.
Scrum half Cillian Willis ghosted down the wing, Chris Henry continued the move and his fellow back rower Thomas Anderson got his name on the scoresheet, taking the try-scoring pass. O'Connor nailed the conversion for a 24-0 interval lead.
It was Nagusa again that set the scoring off in the second half, cantering through for a seven-pointer after the Viadana defence was caught napping once again.
The Ireland-capped Matt McCullough set up Ulster's sixth try in the 55th minute, with Nagusa completing his hat-trick.
Phoenix Ulster Rugby Academy player Jonny Shiels broke through just five minutes later, for a deserved try which made it 41-0.
The Ulster tries continued rolling in - late efforts from McCrea and Academy member Jamie Smith sandwiched Viadana's only try of the game, a 70th-minute score from replacement Antonio Deni.