The representation theory of deformed oscillator algebras, defined in
terms of an arbitrary function of the number operator N, is developed
in terms of the eigenvalues of a Casimir operator C. It is shown that
according to the nature of the N spectrum, their unitary irreducible r
epresentations may fall into one out of four classes, some of which co
ntain bosonic, fermionic or parafermionic Fock-space representations a
s special cases. The general theory is illustrated by classifying the
unitary irreducible representations of the Arik-Coon, Chaturvedi-Srini
vasan, and Tamm-Dancoff oscillator algebras, which may be derived from
the boson one by the recursive minimal-deformation procedure of Katri
el and Quesne. The effects on non-Fock-space representations of the mi
nimal deformation and of the quommutator-commutator transformation, co
nsidered in such a procedure, are studied in detail.