Sebastian Koh started his journey as a musician when he joined the Military Band and played the Euphonium when he was a student in St. Patrick's School. He was later introduced to the trombone by Brando Tan when he was 15. Upon graduation, Sebastian furthered his interest in music at the Nanyang Academy of Fine Arts where he played in the school’s Chamber Orchestra and Wind Ensemble.

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During his National Service, Sebastian travelled to India and Malaysia to participate in military tattoos as a musician of the Singapore Armed Forces Central Band. He was also part of the fanfare team that performs exclusively for the President of Singapore. 

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Following his stint with the SAF Central Band, he returned to the Nanyang Academy of Fine Arts, where he graduated with a Bachelor of Arts from the NAFA-University of Wales Programme. As an undergraduate, Sebastian studied trombone and conducting under the tutorship of then Assiociate Principal Trombone of the Singapore Symphony Orchestra, Fredi Sonderegger and then Head of Conducting and Orchestral Studies, Volker Hartung respectively. After graduation, he continued to study with then Bass Trombone of the SSO, Shannon Pittaway. Sebastian has also performed for the well-known brass mouthpiece designer, Denis Wick.

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Sebastian was part of the Anglo-Chinese School Wind Orchestra that clinched 1st and 2nd prize in 2014 and 2018 respectively, at the Singapore International Band Festival (Open Division). In 2019, he performed alongside renowned film score composer Hans Zimmer and the Metropolitan Festival Orchestra at the Formula 1 Singapore Grand Prix. Sebastian had also appeared on several live Mediacorp shows with Suria and Vasantham. He continues to perform regularly with The Metropolitan Festival Orchestra, The Philharmonic Winds, Lion City Brass Band, and The Swiss Alpine Lions.

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As of date, Sebastian is also a band director at Ang Mo Kio Secondary School and Saint Andrew's Secondary School.