This study structures an adaptive prescribed performance output feedback controller for hydraulic systems with large uncertainties including disturbances and parametric uncertainties. Adaptive control is structured to approximate real system parameters. Based on estimated parameters, a nonlinear disturbance observer for largely mismatched disturbance and an extended state observer for matched disturbance and unmeasurable system states are integrated. Then, to guarantee prescribed tracking performance, an output feedback controller is proposed with uncertainty compensation based on the funnel function using the backstepping method. In addition, to avoid the occurrence of complex analytical calculations and "complexity explosion" in the backstepping method, a differentiator is adopted. Then, the Lyapunov method is utilized to prove the stability of the closed-loop system with estimation errors. Finally, experimental results were obtained to demonstrate that the proposed controller is valid.