The O.S. codex is a program that decodes secret American communiques, particularly those of Operational Support.
While saving the President of the United States from a duplicate, the then Action Man, Mike Brogan was able to acquire a copy of the O.S. codex for the United Kingdom, to serve as leverage against their ally should the need ever arise.
This copy was kept at Hauteville House in Guernsey. When apparent separatist extremists threatened the security of the codex, Ian Noble, the new Action Man, was sent in to retrieve it, which he succeeded at. Unbeknownst to him, this was part of a plan by Mercy Gale, alias Doctor X, to acquire the codex, which she would use in conjunction with IP addresses from American agents to gain access to valuable American intelligence, including the use of alien technology. This plot was ultimately foiled when Ian shot at the data card containing the codex, damaging it beyond the point of usability.