The Office of Secret Intelligence (OSI) is an intelligence agency answerable only to the Emperor, tasked. Among other tasks befitting a secret agency. With the capture of C.C. its members appear not to have been informed by C.C.'s immortality, and only a few selected agents know of the true power of Geass.
The Agency in 2018 was tasked by Charles zi Britannia to monitor Lelouch vi Britannia and search for the whereabouts of C.C. After the tampering of Lelouch's memory by Emperor Charles, Lelouch is kept under constant surveillance without the student ever noticing, a testament to their secretive nature. The Agency also kept the detailed information of what Lelouch did in a journal, which one member of the OSI described as a "Breeding Log" and described Lelouch as nothing more than "bait" or a "trap" for he was used to lure C.C. out and capture her.
In the entrance to the OSI's headquarters, it is opened by a bookshelf within the Ashford Academy. Most OSI Agents are disguised as Ashford Academy teachers or members of the school staff. Some OSI Agents are also disguised as civilians.
Lelouch managed to infiltrate the headquarters and also convinced Rolo Lamperouge to join his side. With Rolo's help, Lelouch successfully managed to blackmail Villetta Nu in exchange for his silence. From then on, Villetta was forced to serve Lelouch and he geassed several OSI Agents. Because of this, Lelouch had the entire OSI under his command and it is unknown of the OSI's status after Lelouch became 99th Emperor of the Holy Britannian Empire.