The Nigeria international helped the Naples club seal Champions League qualification, as the Luciano Spalletti Spalletti side already fell off from title race.
Osimhen, Kalidou Koulibaly, Hirving Lozano, Amir Rrahmani and Dries Mertens (brace) all scored to seal the big win which landed them a place in the continent.
Osimhen, who bagged an assist in the game, also had the ball in the net a second time on 71 minutes, but it was ruled off for offside.
Napoli will face Torino next Saturday as the season draws closer and closer to a close.
Ripples Nigeria recalls that Napoli failed to secure a Champions League place this season after losing the spot to Juventus in the final game of last season.