A disrupted oppositional based gravitational search algorithm (DOGSA) tuned sliding mode controller (SMC) is proposed in this study for the solution of automatic generation control of interconnected multi-area power system under deregulated environment. The novelty of the control scheme is established by performing the sensitivity analysis under different conditions such as variation of the system load, turbine time constant, governor time constant and tie-line power coefficient. The dynamic response of the system under consideration is also studied and analysed in the presence of non-linear constraints namely generation rate constraint with reheat steam turbine, governor deadband and time delay during signal processing. Further, in order to validate the effectiveness of the proposed DOGSA tuning over the genetic algorithm and differential evolution tuned schemes reported in the literature, it is also employed to tune the integral (I), proportional-integral (PI), integral-derivative (ID) and proportional integral derivative (PID) controllers. Moreover, the performance of the optimised SMC scheme is also compared with I, PI, ID and PID controllers. The comparative results reveal that SMC scheme tuned using DOGSA gives better results than the conventional controllers.