An orbital angular momentum (OAM) technology, which uses twisted helical phase structure laser modes to carry multiplexed information, has shown excellent potential to improve the capacity of the free space optical communication (FSO) system under atmospheric turbulence (AT) conditions. A significant challenge in an OAM-based FSO transmission system is the optical signal power fading and induced crosstalk due to abrupt changes in atmospheric turbulence. This research paper, presents a design of a hybrid, spatially multiplexed MIMO-FSO transmission system that incorporates the features of 16-QAM, OAM, and OFDM techniques with spatial mode diversity (SMD) to achieve high transmission rates and channel capacity with reduced power penalty during mitigating the multipath fading effects in different turbulent atmospheric channel conditions. The simulation based results illustrate that the hybrid, spatially multiplexed MIMO-FSO system achieves superior BER performance for the transmission link of 2 km with a forward error correction (FEC) threshold limit of $3.8 \times 10^{-3}$ . The Gamma-Gamma (GG) turbulent model is used to analyze the system performance under various atmospheric turbulence conditions in terms of the optical signal to noise ratio (OSNR), number of subcarriers, OAM states, channel capacity, and power penalty. Comparing with the OAM-multiplexed and OFDM-based FSO transmission system, the capacity performance of the proposed system is significantly improved, and the average improvement is obtained at 650% and 856.04% respectively, at 10 dB OSNR. Furthermore, the result clearly showing a 1.5 dB reduction in power penalty with an increase in transmitter lens aperture at a fixed lateral displacement (LD) of 1.5 mm. For large transmitter beam diameters, the power penalty analysis shows an increase in LD tolerance and a decrease in receiver angular error (RAE). A large numerical value of mode spacing leads to a higher-order OAM state, which leads to high power loss but less inter-channel crosstalk due to beam divergence.