The ordinary-extraordinary-Bernstein (O-X-B) double conversion is considered and the O-X conversion simulated with a kinetic particle model for parameters of the TJ-II stellarator. This simulation has been done with the particle-in-cell code, XOOPIC (X11-based object-oriented particle-in-cell). XOOPIC is able to model the non-monotonic density and magnetic profile of TJ-II. The first step of conversion, O-X conversion, is observed clearly. By applying some optimizations, such as increasing the grid resolution and number of computational particles in the region of the X-B conversion, the simulation of the second step is also possible. By considering the electric and magnetic components of launched and reflected waves, the O-mode wave and the X-mode wave can be easily detected. Via considering the power of the launched O-mode wave and the converted X-mode wave, the efficiency of O-X conversion for the best theoretical launch angle is obtained, which is in good agreement with previous computed efficiencies via full-wave simulations. For the optimum angle of 47 • between the wave-vector of the incident O-mode wave and the external magnetic field, the conversion efficiency is 66%.