Board Thread:Questions and Answers/@comment-108.89.197.12-20140619225128/@comment-4796846-20140619232652

It's the Philanthorpy symbol.

''The kanji symbol is called an "Otakon," a portmanteau of Otacon's name and the Japanese word for "spirit." The exact word is a mix of "Ota" in Katakana and then the Kanji "tamashi" which means spirit (オタ魂). It appeared on equipment designed by Otacon, including Snake's OctoCamo suit, together with the words "to let the world be."''

http://metalgear.wikia.com/wiki/Philanthropy