This paper presents a compact, high-efficiency, mode-switchable dual-band orbital angular momentum (OAM) antenna operating simultaneously at 5.8 and 28 GHz for integrated microwave and millimeter-wave (mm-wave) wireless systems. The proposed design features a single-layer, shared-aperture structure composed of two concentric uniform circular arrays (UCAs), each capable of generating OAM modes with topological charge l = ±1. A symmetric dual-port excitation strategy enables OAM mode switching by selectively exciting one port while terminating the other with a 50Ω matched load, thus avoiding the need for external phase-shifting networks. Full-wave simulations and experimental measurements confirm the successful generation of conjugate OAM modes at both frequency bands, demonstrating effective impedance matching, inter-port isolation better than 30 dB, and well-defined spiral phase wavefronts with boresight nulls in both near- and far-field regions. Compared to existing designs, the proposed antenna offers a compact form factor, simplified implementation, and improved suitability for integrated microwave and mm-wave applications.