Potential mid-frequency oscillation risks associated with offshore wind power integration via a voltage source converter based high voltage direct current (VSC-HVDC) transmission system are addressed. China's first offshore wind VSC-HVDC project, the Rudong project, is taken as the research subject. A mid-frequency oscillation suppression strategy based on virtual damping is proposed. Firstly, the mid-frequency impedance model of the offshore VSC converter is established. Through theoretical derivation and frequency scanning, it is verified that the converter exhibits inductive and positive-resistance characteristics in the mid-frequency band. Combined with an analysis of the capacitive and negative-resistance characteristics of the wind farm in the mid-frequency band, the risk mechanism of 320 Hz oscillation induced by their interaction is revealed. Based on this, the proposed strategy is implemented. Mid-frequency harmonic currents are extracted using a DC-blocking component and a narrow band-pass filter with an adjustable center frequency. These currents are then fed into a virtual damping block, where a reverse suppression voltage is generated. Consequently, the positive damping characteristic of the VSC-HVDC system is reshaped within the target frequency band. Simulation results and field tests demonstrate that oscillations can be rapidly and effectively suppressed by the proposed strategy, while the steady-state and dynamic performance of the system are maintained. Throughout the three years of project operation since commissioning, no oscillations have been reoccurred. A replicable technical solution is thereby provided for offshore wind power VSC-HVDC transmission projects.