In this paper, a new decentralized adaptive output feedback variable structure control scheme is designed for mismatched uncertain large-scale systems where the exogenous disturbance is unknown. The proposed approach uses output information completely in sliding surface and controller design. Therefore, conservatism is reduced and robustness is enhanced. Furthermore, the reduce order system in sliding mode is asymptotically stable under certain conditions. Finally, a numerical example is used to demonstrate the efficacy on the method.