The rampant growth of offshore wind energy in Europe and overseas is calling for advanced operation and maintenance simulation tools for optimal infrastructure asset management. The operational downtimes due to random failures, repair times or adverse weather conditions, greatly reduce the availability of the wind farm and therefore increase the Levelised Cost of Energy of this important source of renewable energy. This paper presents a sensitivity analysis to explore the factors influencing the most the operational availability of an offshore wind farm over a particular geographical area. The analysis is carried out using a newly developed open-access tool for operation and maintenance simulation named as “OpenO&M”. The failure rates of wind turbine components and the weather safety allowable limits for maintenance vessels emerge as critical parameters for wind farm availability, specially in remote areas.