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We explore the potential of combining the advantages of multiple-input multiple-output (MIMO)-based spatial multiplexing with those of orbital angular momentum (OAM) multiplexing to increase the capacity of free-space optical (FSO) communications. We experimentally demonstrate an 80 Gbit/s FSO system with a 2×2 aperture architecture, in which each transmitter aperture contains two multiplexed data-carrying OAM modes. Inter-channel crosstalk effects are minimized by the OAM beams’ inherent orthogonality and by the use of 4×4 MIMO signal processing. Our experimental results show that the bit-error rates can reach below the forward error correction limit of 3.8×10^−3 and the power penalties are less than 3.6 dB for all channels after MIMO processing. This indicates that OAM and MIMO-based spatial multiplexing could be simultaneously utilized, thereby providing the potential to enhance system performance.