Of course, even if Osimhen extends his contract with Napoli, he is still possible to leave Napoli if he doesn't feel comfortable there. Anyway, his current contract will expire in 2025, I still didn't hear a valid information that he already extended his contract with Napoli. If he didn't extend his contract, it will be easier to leave Napoli because he will turn to be a free agent player in 2025.
Real Madrid and PSG seems to be the realistic destination. I personally doubt that Osimhen wants to join Chelsea. If Real Madrid is serious to sign him, I assume Osimhen will prioritize to move to Real Madrid. Even if they sign Mbappe, Osimhen still can join Real Madrid if there is no problem with his transfer fees. The main problem is only about nonsense transfer fees expected by Napoli.
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.
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.
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.