The purpose of this thesis is to build a new condition-based opportunistic maintenance (CBOM) framework which combines short-term hydropower operation scheduling (STHS) and generator maintenance scheduling (GMS). It presents the challenges and limitations of current hydro maintenance research, the state-of-art of hydro generation and optimization in Norway. With the existing STHS framework, the CBOM framework supplements the requirements of building failure model and CBOM model. The generator PLANT004_G1 in the cascaded hydro system is used as the research example. The CBOM model finally schedules 9 maintenance activities in one year for the generator. The sensitivity analysis of the CBOM model shows that it has enough flexibility and can be adjusted according to the maintenance requirements. Among all the parameters, accident penalty and maintenance duration do not influence the maintenance results. The alert level and the upper OM threshold influence the number of maintenance activities. The latter also affects the value of accumulated profits. It is proved that the new CBOM strategy cancels or postpones many unnecessary maintenance activities and is more profitable than age-based maintenance and corrective maintenance.