Olav Anton Thommessen was born in Oslo and, as a child of diplomats in the foreign service, he grew up largely abroad, a fact that influenced his life and work. He attended schools in Germany, England and the United States.
His formal music education in music began in 1964 where he attended the Westminster ChoirCollege in Princeton, New Jersey. The next year he began composition studies at the Indiana University School of Music under Bernhard Heiden and Iannis Xenakis. He graduated in 1969 with a Bachelor of Composition degree. His studies continued in 1970 on a Polish state scholarship to study at the Music Academy in Warsaw under Piotr Perkowski. In 1971/2 he followed the courses at the Instituut voor Sonologie in Utrecht, the Netherlands under Werner Kaegi and Otto Laske.
From 1972 onwards, he distinguished himself by having his works performed at festivals and by well known performers and he also received several commissions.
Thommessen has been a member of the board of Ny Musikk - Norwegian Section of ISCM, the Norwegian Society of Composers and has sat on the State Music Council. He was one of the instigators for the establishment of the Norwegian Music Information Centre and was its chairman of the board during the centre's important first years 1979-85.
Olav Anton Thommessen is a composer and a cultural personality of distinction. He is considered to be among the most important Norwegian composers and he has received commissions from all the larger Norwegian orchestras and festivals as well as from institutions abroad. His great knowledge of music history and film music has made him a much sought after teacher, lecturer and an subject of numerous interviews, he holds a professorship in composition at the Norwegian State Academy of Music, where he has taught since 1972.