In this paper, a general optical wireless communication (OWC) system impaired by atmospheric turbulence is studied. It is shown that if the geometry of the overall OWC system lies in the near-field regime, it can benefit from the advantages of a full multiple-input multiple-output (MIMO) system whether it is implemented by monolithic or sparse apertures. A unified framework is presented to analyze such MIMO OWC systems and their achievable diversity and multiplexing gains are estimated. Moreover, assuming that the size of OWC transceivers and the number of utilized degrees of freedom are constrained, the performance of MIMO OWC systems using monolithic or sparse apertures are compared. The results show that the multiplexing systems with monolithic apertures outperform those with sparse apertures in weaker turbulence conditions but it is expected that their superiority vanishes in the presence of strong turbulence. In terms of spatial diversity, the two design approaches provide similar gains.