This study evaluates the effect of application of spatial diversity technique, namely multiple-input multiple-output (MIMO), on the outage performance of free-space optical (FSO) communication systems operating in atmospheric turbulent medium for both slant path (downlink and uplink) and horizontal links. FSO communication links cover ground-to-high altitude platform system (HAPS), HAPS-to-ground, and HAPS-to-HAPS configurations. System model includes the combined effect of attenuation (resulting from absorption and scattering), angle-of-arrival (AOA) fluctuations, atmospheric turbulence, and pointing error. Atmospheric turbulent channel is characterized by Gamma-Gamma distribution. The Hoyt and Rayleigh distributions are used to model pointing error and AOA fluctuations, respectively. The performance of MIMO FSO communication links is presented depending on the various environmental and system parameters. Results show that the MIMO application in both transmitter and/or receiver can be a prominent alternative concerning the performance improvement for FSO communication links.