The 25-year-old, who joined Galatasaray on loan from Napoli in the summer, headed home Gabriel Sara’s perfect cross in the 67th minute to double the hosts’ lead before Besiktas pulled one back in stoppage time.
The Nigeria international has now scored in three consecutive league games, following his spectacular overhead kick against Antalyaspor last weekend on his return from injury.
The victory saw Galatasaray consolidate their position at the top of the Turkish Super Lig standings with 28 points from 10 matches, opening a six-point gap ahead of second-placed Samsunspor, while Besiktas remain fourth with 20 points.
Okan Buruk’s side have maintained an impressive run of form, winning eight and drawing two of their last 10 matches across all competitions, scoring 32 goals and conceding 12.
Osimhen, who had registered three goals and four assists in just five appearances before Monday’s game, has justified his high-profile loan move to the Turkish giants, with Chelsea reportedly monitoring his progress ahead of a potential move in 2025.
The former Lille forward’s consistent performances for Galatasaray have drawn praise from several former Chelsea players, including John Mikel Obi and Didier Drogba, who believe he would be an ideal signing for the Premier League club.