[2] They picked the red and white colors to honor the football traditions of Sfântu Gheorghe and chose the name OSK as an acronym reminiscent of the defunct Olt Sport Klub.

[2] In order to be able to enroll to Liga V they signed a collaboration agreement with Clubul Sportiv Școlar from Sfântu Gheorghe and formed a junior team.

[8][9] As a last step in becoming fully professional in 2017, the club started a series of investments into infrastructure which included the creation of training grounds for its youth center and building a UEFA Category 4 stadium.

In the years which followed the promotion, Sepsi OSK managed to stay in the Liga I, qualifying for the first time to the championship play-offs in the 2018–19 season.

In the summer of 2020, Sepsi reached the Cupa României final for the first time, but lost it 0–1 to FCSB at the Ilie Oană Stadium in Ploiești.

On their route to the last game of the competition, Sepsi eliminated Ripensia Timișoara, Astra Giurgiu, Petrolul Ploiești and Politehnica Iași; in all but one of the five matches "the Székelys" scored at least three goals.

Sepsi was drawn against Slovakian side Spartak Trnava in the second round of the UEFA Conference League, but were eliminated due to a 3–4 penalty shoot-out loss after two draws.

[12] In comparison to other Liga I teams at the end of 2019, Sepsi OSK stood out with by far the highest revenue obtained from corporate sponsorship deals and having the least debt.

[13] Some of the companies that have or had sponsorship deals with Sepsi OSK include Hungarian firms Gyermelyi, OTP Bank and MOL.

[19] Ownership of the stadium was transferred from the Romanian Ministry of Youth and Sport to the City Council of Sfântu Gheorghe in 2019.

[20] The construction of a new UEFA Category 4 stadium with a capacity of 8,450 seats was started on a lot near Sepsi Arena in the summer of 2018.

[22] Being the first team from Covasna County—a county with a majority of ethnic Szeklers/Hungarians—to achieve promotion to the top tier of Romanian football, Sepsi OSK is widely supported among Hungarians.