This paper presents a decentralised control approach for the hydraulic motor torque provided by a hydrostatic transmission. Based on a control-oriented model of the hydrostatic transmission, a quasi-linear state-space representation with both state-dependent and input-dependent matrices is derived. Using extended linearisation techniques, a combined feedforward and feedback control is designed. Furthermore, a sliding-mode observer estimates unmeasurable states as well as disturbances. The estimates for the disturbances – an external load torque and a leakage oil volume flow – can be used for a disturbance rejection. The proposed overall control structure is investigated thoroughly in simulations and, afterwards, implemented as well as validated on a dedicated test rig.