Currently, radio OAM waves have broad applications because they have natural orthogonality. In the present paper, we proposed an optimal design of a concentric circular array with multiple rings for generating OAM beams with flat top characteristics. Firstly, we introduced a compact OAM antenna using a sequentially rotated configuration and a cost function of the flat-top pattern. Then, an optimization process is designed and conducted, including three independent simulations of sub-arrays, and rooting of excitation coefficients and rotation angles for individual rings. Simulation results show that the optimized antenna can generate an OAM beam with a ripple coefficient of less than 0.6dB on the torus from -55 to 55 degrees, which meets the requirements of flat-top beams.