In 1969, Olsen attracted notice as a guest artist playing the patter roles in the Australian Opera's productions of Gilbert and Sullivan's H.M.S.
[1] Olsen rejoined Australian Opera in 1971–72, playing Ko-Ko in The Mikado and the Duke of Plaza-Toro in The Gondoliers in Sydney, Melbourne and Canberra.
Olsen sang the role of the Lord Chancellor in the production of Iolanthe in the opening performance of the Lyric Opera of Queensland, in Brisbane in 1984.
His other Gilbert and Sullivan roles have included Robin Oakapple in Ruddigore, the Learned Judge in Trial by Jury, Jack Point in The Yeomen of the Guard and Don Alhambra in The Gondoliers.
He was awarded a Churchill Fellowship in 1985 to study operetta production in Europe, and he subsequently directed many stage productions, including Gilbert and Sullivan operas, Countess Maritza and The Czardas Princess by Emmerich Kálmán, and the Mozart operas The Abduction from the Seraglio and Così fan tutte.
[1] In 1991, Olsen appeared as George Grossmith (the original Gilbert and Sullivan comic lead) in a one-man show called A Song to Sing, O, which had been written for John Reed in 1981.
He also appeared in their concert production, An Evening with Dennis Olsen, to celebrate the 125th anniversary of the Law Society of South Australia in 2004.