Osimhen has now scored in his last three league games and has six goals in eight outings in the Italian topflight.
Italian-born Nigerian right-back Michael Kayode featured for Fiorentina which was his fifth league game.
The 19-year-old was taken off with seven minutes left in the game.
Josip Brekalo opened the scoring in the 7th minute before Osimhen level the score from the penalty spot five minutes into first half added time.
Giacomo Bonaventura put Fiorentina 2-1 ahead on 63 minutes while Nicolas Gonzalez added the third for the visitors in the 93rd minute.
Napoli are 5th on 14 points and are seven points behind leaders AC Milan in the league table.
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