This paper presents an Extended State Observer (ESO) based anti-disturbance controller for Omnidirectional Mobile Robots (OMRs) subject to uncertainties and wheel skidding. The ESO is utilized to recover the velocities and estimate the total disturbances consist of uncertainties, external disturbances and skidding. Then the disturbances are compensated by an anti-disturbance controller proposed with backstepping technique. The Input-to-State Stability (ISS) of the closed-loop system is analyzed via cascaded theory and simulation results are provided to verify the effectiveness of the proposed controller.