Earlier, he had won the Golden Boot award at the 2015 FIFA U-17 World Cup, straight from his Ultimate Strikers Academy, based in Lagos, which Nigeria won. He made his senior international debut in June 2017 and played at the 2019 Africa Cup of Nations. He is currently the ninth all-time highest goalscorer of the Nigeria national team.
Osimhen earned his full Bundesliga debut on 28 January 2018 against Hannover, playing the full 90 minutes in their 1–0 win. Osimhen made two more starts the rest of the campaign, playing the full match against Werder Bremen on 11 February
On 22 August 2018, Osimhen joined Belgian club Charleroi on a season-long loan deal. He made his debut on 1 September against Excel Mouscron, entering as a late substitute for Jérémy Perbet. Osimhen made his full debut on 22 September, scoring his first goal as a professional with a backheel against Waasland-Beveren. He also scored twice in the last five minutes in a 3–2 win over Zulte Waregem on 21 October, after Hamdi Harbaoui had given Waregem a 2–1 lead with a second half brace. Osimhen was not dropped from the side until 25 November, when he scored a goal off the bench in the club’s 4–2 win over Lokeren. After the game, Osimhen told BBC Sport that he had “found his happiness again”. Osimhen finished the first half with 8 goals in 16 games, second only to Cristian Benavente in the team.
After a successful spell with the Belgian side, playing 36 games and scoring 20 goals, Charleroi activated their option to acquire Osimhen following his satisfactory performances while on loan.
In July 2019, Osimhen signed for Lille. On August 11, 2019, he made his Ligue 1 debut for the club, scoring a brace in a 2–1 win against Nantes.
Osimhen was named the Ligue 1 Player of the Month for September 2019, having scored two goals and assisted two more in Lille’s five league matches. On 2 June 2020, he was awarded Lille’s Player of the Season, having secured the highest number of votes from fans.
Osimhen scored his first goal in the UEFA Champions League on 2 October 2019, coming in a 1–2 home defeat to English side Chelsea. The Nigerian finished the season as Lille’s top scorer, with 13 goals in 25 Ligue 1 matches, and 18 goals across all competitions. His strong goalscoring performances saw Osimhen tipped by many to be in running for the African Footballer of the Year award in the near future.
On July 31, 2020, Serie A club Napoli announced the signing of Osimhen, for a club-record fee of €70 million potentially rising to €80 million with add-ons, making him the most expensive African transfer to date.
On October 17, 2020, Osimhen scored his first goal for Napoli in a 4–1 win against Atalanta. Upon scoring, he held up a shirt calling for the ending of the ongoing police brutality in his home country of Nigeria.
While recovering from the shoulder injury, Osimhen contracted the COVID-19 virus, as he tested positive upon returning to Naples. This positive test, together with his shoulder injury, kept him away from action until his return to the side on January 29, 2021, when he was subbed on for around twenty minutes of Napoli’s Coppa Italia win against Spezia.
On October 29, 2022, he scored his first Serie A hat-trick in a 4–0 win over Sassuolo.
On November 11, 2022, he scored a goal and assisted another to help Napoli win 2–1 at Atalanta, becoming Nigeria’s highest goal scorer in the Italian Serie with 32 goals, and surpassing Simy’s record of 31. On 13 January 2023 Osimhen scored two and assisted another as Napoli beat Juventus 5-1.
On February 17, 2023, he scored his 100th career goal to help Napoli win 2-0 at Sassuolo. He also became the first-ever player in the history of Napoli in the three-point era to score in seven league games in a row.
