Otto Olsson (1879–1964), professor of organ at the Conservatory ofthe Royal Academy of Music, Stockholm, and a prolific composer of organ and choral music in particular, was the first – and for a long while, only – Swedish composer to use Gregorian and other medieval church melodies in organ music. Olsson used plainsong material in two collections of short pieces (Gregorianska melodier and Six Pieces on Old Church Songs) as well as in two larger works (Ten Variations on the Dorian Plainsong ‘Ave Maris Stella’ and the second organ symphony, Credo Symphoniacum), all composed between 1910 and 1918. Olsson’s use of plainsong themes, largely unknown in Sweden at the time, can be related to international developments in church music aesthetics originating in the nineteenth century, which saw an increasingly important distinction between sacred and secular music. Both the use of melodies from the Old Church and their musical setting were central to Olsson, whose goal was to create music that could be recognized as ‘church music and nothing else’. Olsson’s settings vary considerably in form and character, ranging from free recitative to strict polyphony, and even attempts to combine the two approaches. The free treatment of the chant melody in the Ave maris stella variations (probably the first of his ‘Gregorian’ works) stands in contrast to the unchanged cantus firmus in the other compositions. In most pieces, text-related musical symbolism is not prominent, the exception being Credo Symphoniacum, in which the composer makes varied use of eight melodies to represent the Christian Creed and the Trinity.