We propose an OAM-MIMO multiplexing system that effectively combines orbital angular momentum (OAM) and the advantage of multiple input multiple output (MIMO) based digital signal processing with multiple uniform circular arrays (UCAs) for point to point (P2P) line of sight (LOS) wireless transmission. We also implement transmitting (Tx) and receiving (Rx) antennas with quadruple UCAs, and each UCA has a broadband Butler matrix circuit for generating and separating OAM modes in the 28-GHz frequency band. We experimentally successfully demonstrate 120-Gbps wireless data transmission using five OAM modes (0, +/- 1, +/- 2) and 2-Gbaud adaptive modulation and coding with maximum modulation order of 256-QAM in the 27.5-29.5-GHz range. Our experimental results show the first step toward the next stage for super-high-capacity wireless communications.