In this paper we consider first the output feedback disturbance decoupling problem for linear systems. We derive a wt of new necessary or sufficient conditions for the solvability of this problem. These conditions are expressed in terms of system structural invariants such as infinite zero orders and essential orders. Then we consider the output feedback disturbance decoupling problem for structured linear systems. Necessary or sufficient conditions for the generic solvability of this problem are provided. They are expressed in terms of input-output path length comparisons on the system-associated graph.