Well Napoli average 61.3% possession per game within Serie A. For context if a PL team averaged that they'd be second only to City. In contrast we've averaged 52.6% so far this season. So they certainly have a possession-friendly style of play and Osimhen fits within that.
In terms of contributing to it though, Osimhen doesn't particularly get involved in general play, being second bottom in Napoli's squad for passes attempted per 90 with around 12. Essentially his function in those possession phases is largely to occupy the opposition defensive line and force them deeper by being a central off the ball threat via pace, physicality and movement. With that obviously affording his teammates more space.
Haaland at City is probably the closest comparison in that sense, with him being similarly uninvolved on the ball for a possession-heavy side, bottom for passes per 90 in their squad with around 15. With Osimhen not offering quite the sheer goal threat Haaland does, but offering more in terms of pressing ability and workrate.