Control input signals are usually transferred to real applications with an inevitable transmission delay. On the other hand, most of the nonlinear models, which are compatible with industrial processes, include constrained inputs and singular control gains. Therefore, designing a robust control approach, which is compatible with limited inputs and can overcome singular gain, input delay and external disturbances is crucial. In this study, an observer-based adaptive fractional order sliding mode control (OBAFOSMC) is proposed for a class of nonlinear delayed systems with singular control gain and external disturbances. To this end, firstly an adaptive fractional order sliding mode observer is designed to estimate the virtual disturbance induced by the singularity in the system, through which the estimation error converges to zero exponentially fast. Then, an adaptive fractional order sliding mode control is designed for the predicted nonlinear system to control the states to desired values exponentially. To the authors’ knowledge, this is the first time that an adaptive fractional order sliding mode control is proposed for a nonlinear delayed system with constrained input signals and singular control gain. Analytical results through Lyapunov theorem demonstrate that the designed method effectively stabilizes the system and avoids chattering phenomenon via adaptive smooth functions. Finally, the simulation results confirm the effectiveness of the analytical attainments.