In recent years, offshore wind power development is expanding from nearshore areas to deep-water and far-offshore areas, creating operating scenarios where nearshore wind farms are connected through AC transmission and far-offshore wind farms through VSC-HVDC transmission, with both systems integrated at an onshore grid-connection point. The interaction between AC-connected wind farm and VSC-HVDC converter may affect system oscillatory stability. Based on the actual situation of a certain project, a calculation example system of offshore wind farms integrated with combined AC and VSC-HVDC is established and the state space model of the case system is developed. Based on the linearized state space model, the dominant eigenmodes of the system and their instability scenarios are analyzed. By comparing the variations in the dominant eigenmodes of AC-only, VSC-HVDC-only, and combined AC/VSC-HVDC systems under different transmission-capacity allocations, the key factors affecting the oscillation characteristics of offshore wind power systems with combined AC and VSC-HVDC grid integration are identified. Finally, the correctness of the analysis results is verified through detailed electromagnetic transient simulations. The research conclusions provide guidance for the planning and operation of offshore wind power with combined AC and VSC-HVDC grid integration.