Omar Anton Pedersen Gjesteby (4 June 1899 – 29 October 1979) was Norwegian trade unionist and politician for the Labour Party.
He was born in Berg, Østfold as a son of farmers Olaf Pedersen Ugjesteby (1867–1918) and Anette Elise Grasmoen (1868–1954). He finished commerce school at Treider in Kristiania in 1915, and worked in a shop, in a warehouse and with newspaper sales between 1915 and 1926. He was a board member of the trade union Avis-, bok- og papirfunksjonærers forening, and from 1926 he worked as a secretary in the trade union Union of Employees in Commerce and Offices. He was also active in the International Organisation of Good Templars. From 1923 to 1927 he was a national board member of the Social Democratic Youth League of Norway (the youth wing of the Social Democratic Labour Party), and from 1927 to 1929 he was a central board member of the Workers' Youth League.
In the Labour Party he was a supervisory council member in the Oslo branch from 1932 to 1968. He was a member of the executive committee of Oslo city council from 1934, and also a deputy representative to the Parliament of Norway during the term 1934–1936. He also advanced to deputy leader of the Union of Employees in Commerce and Offices.
In April 1940, some days after the German invasion of Norway, Gjesteby showed signs of willingness to cooperate economically with the invaders. However, in November 1940, some time into the German occupation of Norway, he was removed by the Nazi authorities as deputy leader of his union. Shortly after, he was arrested because the Labour Party fraction in the executive committee of Oslo city council criticized the Nazification of the County Governor post. He was later released. He found work as office manager in the fish food company Erling Moe, but had to flee the country in 1944. He fled to Sweden where he worked in Svenska Norgeshjälpen until the end of the Second World War. He was a member of the secretariat-in-exile for the Norwegian Confederation of Trade Unions in .