I am almost sure that Osimhen will leave Napoli. You will have to pay a lot of money for it. Chelsea are ready to pay this money and they can handle this burden financially. The biggest advantage for Chelsea is that Osimhen is very interested in playing in the Premier League. I am sure he will achieve success with Chelsea in the Premier League.
Osimhen base price is actually around €120m, it is according to transfermarkt. But Napoli president always tells us non sense price for Osimhen. It is likely no interest to sell Osimhen to other clubs. The latest news I read, De Laurentiis will only consider an offer above €500 million for Osimhen. I think no club wants to sign him at that price, it is too crazy. Even I doubt Chelsea has an interest to sign Osimhen if Napoli only wants the offer above €500 million.
As far as I know, Osimhen was in the sight of PSG who tried to sign him for something around €200m which failed. That was one of the highest bid for him.
Osimhen is willing to go to EPL, otherwise he would be in a Saudi club or PSG. From what it appears, Chelsea is the most interested to bring him, not sure about Arsenal.
I find any price above €100m for him as abusive, especially with his current form. I believe Napoli could let him go this winter if he continues playing like that..