From Halopedia, the Halo wiki
Core 5 is a section of HIGHCOM Facility Bravo-6 in Sydney, Australia, on Earth. It contains the offices of the staff of the Commander-in-Chief of ONI, formerly Admiral Margaret Parangosky. Core 5 also contained then-Captain Serin Osman's office prior to her ascension to the position of CINCONI, connected to Parangosky's office by a corridor. Osman's office contained an oak book case which held an ancient copy of The Admiralty Manual of Seamanship Volume II, a gift from Fleet Admiral Hood. The Department of Colonial Security reports directly to Core 5.
List of appearances