In this paper we introduce the OCS (Observatory Control System) of the LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope), which will survey more than ten million galaxies and stars to get their spectra after 2004. The OCS will operate the TCS (Telescope Control System) and ICS (Instrument Control System) in real-time to accomplish spectroscopic observations. Each observation could obtain spectra of about 4000 objects simultaneously and the amount of raw data per night is 2 - 3 gigabytes. The OCS will also handle the observational schedules and the data processing, which is called as the DHS (Data Handling System). The propose of the OCS is to make whole observation activity (including object selection, observational scheduling, observation at telescope, data processing, data archiving and so on) automatically and to gain scientific return more efficiently. The OCS is a software system connected with TCS, ICS and DHS by the computer networks. It involves many advanced information techniques, such as network, communication, database, web, GPS, et al.