The Blues have been in hot pursuit of the former Lille man throughout the summer transfer window, and his future remained clouded in uncertainty as the clock ticked down on transfer deadline day.
Paris Saint-Germain were originally believed to have been Chelsea's biggest competitors, but in spite of an injury to Goncalo Ramos, the French champions are not pursuing a deal.
Instead, Chelsea were facing an intense battle with Saudi giants Al-Ahli to sign Osimhen, whom Napoli are apparently ready to sell to the Middle Eastern powerhouses for below his £113m release clause.
However, Osimhen had not given the green light to a move to the gulf state before deadline day, offering Chelsea late hope that a deal can be done for the 25-year-old, whom they were believed to have entered talks over personal terms for.
Al-Ahli 'finalising' £67m Osimhen signing
Osimhen's agent also shot down rumours of an imminent move to Saudi Arabia for his client earlier this week, and it was claimed on Friday morning that the Blues had come forward with a loan to buy offer for the striker.
Enzo Maresca's side will seemingly have to finally concede defeat in the race to sign Osimhen, though, as journalist Gianluca Di Marzio claims that the Nigerian has decided to join Al-Ahli.
Di Marzio reports that both parties are now finalising the last details of a four-year deal for Osimhen, whom Napoli will bank an €80m (£67.3m) for, and the striker will bank a monumental €40m (£33.7m) in Saudi Arabia.
Osimhen was believed to have been stalling on a move to Saudi Arabia due to Al-Ahli's original contract offer not including a release clause, but the club have supposedly now agreed to insert one into his terms.
As a result, Osimhen - whose agent has claimed still has unfinished business in European football - will be able to return to Champions League territory in the future should he wish, and he leaves Napoli with 76 goals and 18 assists under his belt from 133 matches.
Osimhen will link up with the likes of Riyad Mahrez, Roberto Firmino and Edouard Mendy at Al-Ahli, and Napoli have already signed Romelu Lukaku from Chelsea as his successor.
What Osimhen alternatives can Chelsea target?
Mere hours after a loan with an obligation to buy offer renewed hope that Chelsea could pull off the Osimhen deal, the astounding financial package offered by Al-Ahli proved too tempting for the striker to turn down.
Chelsea placed most of their attacking eggs in the Osimhen basket, but not all of them, as the Blues are still thought to be interested in poaching Ivan Toney from Brentford in the closing stages of the window.
The England international has been effectively frozen out of the Bees' squad and was previously linked with a move to Al-Ahli before the Saudi giants hijacked Chelsea's pursuit of Osimhen.
Lille's Jonathan David could represent a cut-price option for Chelsea, but the Blues could very well end up going into September having failed to address their main weak point, despite another significant summer of spending.