According to Steven Best and Douglas Kellner (_Postmodern Theory_):
English painter John Watkins Chapman spoke of 'postmodern painting' around 1870 to describe at-that-time avant-garde work
German Rudolf Pannwitz used the term in a 1917 book to describe European nihilism
British historian D. C. Somervall referred to a 'postmodern' break with the modern age in a one volume edited version of Arnold Toynbee's _A Study of History_ (1947)...Toynbee used the term as well in a 1963 revised edition of the complete work
cultural historians Bernard Rosenberg and David White described mass society as postmodern in their 1957 book _Mass Culture_...economist Peter Drucker subtitled his book of the same year, _The Landmarks of Tomorrow_, 'A Report on the New Post-Modern World'
C. Wright Mills wrote of the modern age 'being succeeded by a post- modern period' in _The Sociological Imaginaion_ (1959)
in 1961, Huston Smith characterized the view that reality is unordered and unknowable as the 'postmodern mind'
the term 'postmodern' was used a bit in the 1940s & 1950s to designate certain developments in architecture & poetry (would include Olson, I think)...term began to be used in the 1960s & 1970s by cultural theorists (as listers, no doubt, already know)...Michael Hoover