[2] In 2018, Oscar Chelimo finished in seventh place in the men's 5000 metres event at the World U20 Championships held in Tampere, Finland.

[3] He also competed in the boys' 3000 metres at the Youth Olympics in Buenos Aires, Argentina and won the bronze medal.

[1] He represented Uganda at the 2019 African Games held in Rabat, Morocco, running in the men's 5000 m event to finish in fifth place.

[1] In 2020, he won the men's 5 kilometres road race at the BOclassic held in Bolzano, Italy.

On 12 May 2021, Chelimo raced the 5000 m in Bergamo, Italy, finishing third in 13:06.79, an Olympic qualifying time.