He studied (among others) with Alois Hába at Prague Conservatoire (1925–27; he also completed Hába’s quartertone music course. After his return to Ljubljana, he was a professor at the conservatoire and later – at the Academy of Music. He played a key role in the development of Slovenian artistic music trends between the world wars. In his pupils, called ‘Ostercians’, he instilled the aspiration to explore modernist means of expression and write modern works. He was also active as a music critic and journalist, writing essays and reviews for Zvuk.
His idiosyncratic musical language drew on the Neo-Baroque, Neo-Classicist and expressionist tendencies, as well as on atonal music and athematicism (a style propagated by Hába, in which melody developed without repetitions and motif transpositions. His music was performed at ISCM festivals (1934, 1935, 1938, and 1939). He was also a member of the international jury programming the ISCM’s Paris festival in 1937. As a progressive leftist intellectual, he most clearly reflected his views in choral works taking up social issues.
It is in Koffler’s letter to Osterc of November 1936 that we find one of the first mentions of the latter artist’s son Alban. Koffler helped Osterc find new contacts in the world of new music, including such figures as the conductors Grzegorz Fitelberg and Fritz Mahler, as well as the Argentinian composer Juan Carlos Paz.
Koffler sent Osterc scores by Polish composers, including Karol Szymanowski. In late 1936, he also made efforts to organise a concert of Yugoslav music at Lwów Philharmonic. Osterc, in turned, endeavoured (unsuccessfully) to include Koffler’s Four French Songs in the programme of the ISCM’s Paris festival in 1937. Koffler’s letters to Osterc are kept at the National and University Library of Slovenia (NUK) in Ljubljana.