This study evaluates the seismic performance of offshore wind turbines (OWTs) under bi-directional earthquake loading, focusing on both fixed-bottom and floating structures. For fixed-bottom OWTs, time-history analyses were conducted to assess dynamic responses of jacket supported OWTs considering two horizontal seismic components. The results were compared to those obtained using the percentage combination rules (30% and 40%) from building codes. It was found that the 30% combination rule reasonably predicts tower top displacement and rotation, with differences of 3% to 25% compared to time-history analysis. Additionally, a fragility analysis of jacketed OWTs was performed to evaluate seismic vulnerability under different damage states based on serviceability and ultimate limit states. The results show that tower-top drift is more vulnerable in the serviceability limit state compared to the ultimate limit state, particularly for low to high levels of earthquake intensity. For floating wind turbines, the study performed a coupled multi-directional seismic analysis of a Tension Leg Platform (TLP) wind turbine. TLPs were found to be highly susceptible to pulse-like ground motions, mainly due to large velocity pulses. Additionally, the variability in responses increased with peak ground acceleration, highlighting the need for larger safety margins in high seismic risk areas. Seismic vulnerability assessments indicate that jacket structures exhibit higher susceptibility to seismic shaking compared to Tension Leg Platform (TLP) floating wind turbines. The study provides valuable insights for improving the seismic design of OWTs, offering targeted risk mitigation strategies to enhance safety and structural resilience in earthquake-prone regions.