The Transition SST turbulence model was used to simulate and analyze the influence mechanisms of O-ring with different relative spacings and inclination angles on the mechanical characteristics of the cylinder surface, vortex shedding characteristics, and wake flow structures at a subcritical Reynolds number. The results show that the inclined O-ring with a relative spacing of 3/4 significantly controls the unsteady hydrodynamic characteristics of flow around a cylinder by extending the shear layer, disrupting the dominant frequency of vortex shedding, reducing the pressure difference between the front and rear of the cylinder, and decreasing the streamwise fluctuating velocity and turbulent kinetic energy behind the cylinder. With the increase of the inclination angle, the control effect is enhanced. Compared with the uncontrolled case, the time-averaged drag coefficient and the standard deviation of lift coefficient are reduced by about 13% and 37%, respectively, and the Strouhal number is reduced by about 7%. The inclined O-ring with a relative spacing of 3/4 and an inclination angle of 30° shows the best control effect, which is the optimal inclined O-ring structure.