Optical spatial modulation (OSM), a new optical multiple input multiple output (OMIMO) technique, effectively improves the transmission rate and energy efficiency by using the spatial domain laser index to carry additional information. Meantime, since only one laser is activated per symbol duration to transmit information, the problems of channel interference and synchronization are solved in traditional OMIMO system. This paper firstly introduces several OSM technologies and summarizes their research status both at home and abroad. Nextly, the OSM, optical space shift keying (OSSK), enhanced optical spatial modulation (EOSM) and differential optical spatial modulation (DOSM) schemes are compared and analyzed in terms of transmission rate, spectral efficiency, bit error rate (BER) and complexity. Finally, the key problems and future development direction are pointed out in OSM system.