In 1945, Fladmoe made his debut as conductor of the National Theatre in Oslo and continued until there until 1959, except for the 1947–48 seasons where he was conductor of the Trondheim Symphony Orchestra. He conducted both the Oslo Philharmonic Orchestra and the Bergen Philharmonic Orchestra and was principal conductor in Bergen from 1958 until 1961. He was the first musical director at the Norwegian National Opera from 1961 to 1973. He also served as professor at Norwegian Academy of Music.