Of course he can leave Napoli if the offer submitted by the team interested in him meets the wishes of the Napoli owner. But if the offer is not suitable, Napoli management could reject the offer, which would mean Osimhen would have to stay at Napoli. If you look at Osimhen's game last season, he was quite diligent in scoring goals, but this season he seems to have experienced a setback in his performance.
Napoli management can reject the offer as long as Osimhen still has a contract with Napoli. But his contract will be over in 2025, Napoli management has no ability to reject the offer for Osimhen when the contract expires in 2025. If it is difficult to leave Napoli now, Osimhen needs to wait for until 2025 and never extend his contract.
In the Napoli vs Real Madrid match where Osimhen was played from the first minute but in this match he was unable to make much of a big contribution to Napoli so he really experienced a decline in his performance.
We can't judge his quality or his performance from a single match. I think Osimhen still has a good ability and performed quite well. He just can't get enough supports in the match against Real Madrid.
In fact, I won't believe that Real Madrid will try to approach him in the winter window. Even though Real Madrid needs a striker, I think they are more interested in recruiting Mbappe than Osimhen.
We don't know what the plan made by Real Madrid.