From Japan SAQ (or Seldom Asked Questions):

Q. What does the name of the famous hamburger chain, 'MOS Burger' mean? A. The 'MOS' in 'MOS burger stands for "Mountain, Ocean, Sun'. According to the official MOS Burger web site, the mountain, ocean, and sun in the name symbolise the company's "infinite love" of both humankind and nature. MOS Burger's love of people and nature is "Grand and noble like a mountain", "Possessed of a spirit as deep and wide as the ocean", and filled with a "passion that, like the heat of the sun, never burns out."And you thought you were just getting a hamburger.