Defender Ronald Araujo came to Barcelona's rescue with a last-gasp equaliser to deny lowly Granada a second successive LaLiga victory at the Nou Camp.
Araujo struck in the final minute of normal time amid a concerted flurry to cancel out Domingos Duarte's early opener and spare his side's blushes as the game finished 1-1.
Jorge Molina went close to a second for the visitors, but Sergi Roberto was denied at the other end by the crossbar and Memphis Depay failed to round off a quick break as the Catalan giants turned the screw.
Araujo, Sergino Dest and substitute Oscar Mingueza all went close with the half-time whistle approaching, but Barca went in at the break still trailing.
Araujo and Depay threatened again as the second half unfolded, but keeper Luis Maximiano and his defenders held firm with substitute Luuk De Jong heading just over from close range with 11 minutes remaining before Maximiano saved superbly from Araujo.
Substitute Gerard Pique was thrown into an attacking role as the home side fought desperately for a way back, and it duly arrived in the final minute of the game when Araujo headed them level in the nick of time.
The visitors took the lead with 24 minutes gone when Osimhen, who had scored both goals as his side fought back to secure a 2-2 Europa League draw at Leicester on Thursday evening, struck from close range after running on to Mario Rui's through-ball.
It might have been 2-0 within seven minutes when Fabian Ruiz's long-range strike came back off the woodwork, but the reprieve proved temporary when defender Amir Rrahmani headed home Kalidou Koulibaly's 35th-minute cross.
Gerard Deulofeu passed up an opportunity to reduce the deficit eight minutes before the break and Koulibaly effectively wrapped up the points within seven minutes of the restart with Napoli's third, substitute Hirving Lozano's late strike simply cementing a comfortable win which eased them from third place to first.