Fly-half Dan Biggar kicked eight points and controlled the ball well to prevent Aironi from putting their tails up and causing an upset - which they threatened to do early in the first half.
Fussell got the first try after just five minutes but Aironi fly-half Naas Olivier knocked over three points in reply soon after.
Good work under the high ball by No.8 Joe Bearman then ignited an Ospreys attack resulting in Biggar atoning for his initial missed conversion by slotting three points.
But the hosts, in their last game of the season, showed they were up for the fight and it was former Auckland Blue Nick Williams at No.8 who nicked a try in the 23rd minute, yet crucially Olivier couldn't convert making the score 8-8.
Five minutes before the break Fussell's back-three colleague Dirksen matched him in flashing across the tryline and Biggar converted.
Olivier narrowed the gap in the 39th minute to make it 15-11 at half-time at the Stadio Luigi Zaffanella.
Aironi started to ring the changes five minutes into the second half.
Head coach Rowland Phillips first called time on full-back Paolo Buso's return to action after more than a year out of the game before throwing front-rowers Alberto de Marchi and Fabio Ongaro onto the park in an attempt to rein back the Ospreys' march.
It had the opposite effect as young prop Lorenzo Romano saw yellow on 58 minutes as the Welsh side cranked the tourniquet at scrum time.
From there Aironi's chances of beating Welsh opposition for the first time diminished as Biggar knocked over another penalty five minutes later.
Flanker Tom Smith was sin-binned for the visitors soon after but they easily saw out the game, in part thanks to replacement Luciano Orquera's wayward form from the kicking tee as the Italian international missed two penalties late on.
The Ospreys can now prepare for hosting the champions Munster - who they've beaten twice this season with their away win at Thomond Park in October ending the champions' 18-month unbeaten home league record.
