Wave power is a promising technology for producing renewable energy. Several types of wave energy converters (WECs) have recently been developed and introduced as technical solutions for capturing the energy of ocean waves. One of the WEC types is an Oscillating Wave Surge Converter (OWSC) that extracts energy from wave surges and the movement of water within them. Reliable operation and high system availability of WECs are key aspects to achieve the levelized cost of energy (LCOE) targets. Systematic reliability engineering methods can offer valuable support for the design and evaluation of highly automated multi-technical WEC systems. Analytical and simulation-based methods such as Failure Mode, Effects and Criticality Analysis (FMECA), Reliability Block Diagrams (RBDs) and Life Cycle Cost (LCC) calculations can be utilized in from early conceptual design phase. The system approach aims to guide the work to right system hierarchy level and to enable the use of available but partly uncertain information for system models, analyses and simulations. The research on reliability engineering methodology in VTT is related to concept development and system design of an OWSC and especially its power take of system (PTO) in an ongoing EU funded research project ‘MegaRoller’. Our results confirm that reliability-engineering efforts should be considered as an interconnected and iterative process, and the system approach helps to look at things at the appropriate level and accuracy.