The problem of disturbance decoupling by means of output feedback is addressed. The notion of conditioned invariance is reconsidered in a linear algebraic framework. Using this geometric notion, a necessary and sufficient condition (respectively, necessary condition) is obtained for solving the disturbance decoupling problem by quasistatic output feedback (QDDPO) (respectively, the disturbance decoupling problem by dynamic output feedback (DDDPO). The necessary conditions for DDDPO which are derived are weaker than the existing ones. A sufficient condition for the disturbance decoupling problem by static output feedback (DDPO) is given as well.