The 21-year-old attacker was handed his third start in the Italian top-flight since his big-money move from Lille this summer.
The centre-forward, who opened his account in his last league outing against Atalanta, could not, however, add to his strike at Stadio Ciro Vigorito.
Gennaro Gattuso’s men put their disappointing defeat against AZ Alkmaar in the Europa League behind them by clinching their second consecutive league victory against the Sorcerers.
The Parthenopeans started the game impressively and completely dominated the opening 30 minutes but failed to make their efforts account.
Benevento then scored from their few opportunities that came their way through Roberto Insigne on the half-hour mark.
Napoli came back into the second-half stronger with Lorenzo Insigne firing into the back of the net but the effort was ruled out for offside by the Video Assistant Referee (VAR).
The forward then made up for the lost opportunity when he equalized with a fine strike on the hour mark after he was set up by Matteo Politano.
Andrea Petagna then scored the match-winning goal seven minutes later to help the Parthenopeans secure their fourth win of the season.
Osimhen featured for the duration of the game and struck two shots, completed one dribble, made 27 touches and had a 73% successful pass rate.
The victory moved the Stadio San Paolo outfit to second spot on the league table after gathering 11 points from five games.
The Super Eagles forward will hope to continue his impressive performances when Napoli square off against Real Sociedad in a Europa League game on October 29.