A smattering of fans, wearing masks and widely spaced apart, watched the game at the Stadio Tardini after the Italian government agree to allow up to 1,000 spectators at matches from Sunday for the first time since the COVID-19 lockdown.
A cagey first half produced no real chances but Napoli came to life after Nigerian Osimhen, signed from Lille in the summer, was brought on in the 61st minute.
Two minutes later, Hirving Lozano crossed the ball into the area and Parma defender Simone Iacoponi headed his attempted clearance straight to the feet of Mertens who fired past Luigi Sepe from 15 metres.
Insigne struck the post from Oshimhen's flick before Parma gifted Napoli a second goal. Lozano intercepted a pass out of the Parma defence, his shot was saved by Sepe but Insigne snapped up the rebound in the 77th minute.
Parma, playing their first match under coach Fabio Liverani, did little to impress new president Kyle Krause, chief executive of the U.S.-based Krause Group which took over the club on Friday.