The former All Black opened the flood gates after just two minutes as the Ospreys helped themselves to their highest score with a record eight tries against the Italian minnows.
Shane Williams also scored twice while Huw Bennett, James Hook, Sonny Parker and Lee Byrne also joined in the try blitz at Stadio Comunale San Michele. The result secured maximum points for the Ospreys and got them back on track following their defeat to Stade Francias last time out.
The Ospreys welcome Calvisano to the Liberty Stadium next Friday night but will be favourites to go top of pool three after this emphatic performance.
They had the bonus point in the bag after just 32 minutes following a devastating start despite the loss of British Lion Ryan Jones to a stomach bug on the morning of the game.
Alun Wyn Jones provided the link as Marshall went over after just 125 seconds before the Ospreys scrum stole the second, shoving their opponents off their own ball for Williams to dart over in the corner. Hook converted both early tries.
Huw Bennett crashed over for his second try in a week following a devastating lineout drive, before Gavin Henson put Hook through for the bonus point score.
Williams revelled as Calvisano tired, creating one try for Sonny Parker just before half-time before spinning out of the clutches of three defenders early in the second half.
Marshall grabbed his second before man-of-the-match Ian Evans showed remarkable kicking skills for a 6ft 8ins lock, chipping over the defence for Byrne to score. Calvisano hit back late on through Alessandro Zanni and Roland de Marigny after the Ospreys rung the changes, but it was mission complete for the visitors.