ONI memorial
From Halopedia, the Halo wiki
The ONI Monument is found inside of the Office of Naval Intelligence. You can also see it in the Firefight map Alpha Site.
Left Plaque
At the top it states:
In Honor of those men and women of Naval Intelligence who gave their lives in service of the UNSC in the War against The Covenant.
Below that it lists the men and womens names.
At the bottom it states:
Their bold initiative, intrepid fighting spirit and unwavering devotion to duty in the face of certain death reflected great credit upon themselves and were in keeping with the highest traditions of U.N.S.C. Naval service. They gallantly gave their lives in defense of the Human Race.
Center Plaque
It states from top to bottom:
ADM Cole, Preston SN: 03956-26127-PC + 04/04/2484--04/18/2543 + In 2525, then Vice Admiral Cole led a battle group to Harvest following the unprovoked attack by the Covenant on that peaceful agricultural colony. After a hard won battle, Cole and the battle group returned to Earth only to learn that other colonies had been destroyed as well. After his promotion to Admiral, Cole led a campaign against the enemy for the next five years, culminating in over a Covenant fleet in 2531.
VADM STANFORTH, MICHAEL SN: 00834-19223-HS + 07/03/2486--08/30/2552 + Vice Admiral Hieronymus Micheal Stanforth was the head of Section THREE of the Office of Naval Intelligence from 2512-2530. Section THREE was at its busiest under his administration many projects were leverage in order to boost support for the war effort (including Project SPARTAN, MJOLNIR and GUNGNIR). In August of 2530 he resigned as head of Section THREE to take a more active role in the defense of UNSC space--leading the UNSC Navy in the Battle of Sigma Octanus IV and directing the Defense of Reach during its final days.
Doctor Catherine Halsey, MD, PhD CI: 10141-026-SRB4695 + 03/19/2492--2552 + Doctor Catherine Halsey is a computer scientist and medical doctor. She has published papers on cybernetics, genetic manipulation and artificial intelligence. Dr. Halsey served as scientific adviser to the UNSC's Office of Naval Intelligence from 2515 until 2522 when she was made Chief Scientist, a position she held until the time of her death in 2552. Dr. Halsey was on Reach at the time of its attack by the Covenant and though no body was recovered she is presumed dead.
Right Plaque
At the top:
In Honor of those men and women of Naval Intelligence who gave their lives in service of the UNSC at Reach
Then it lists the men and women's names.
And at the bottom it states:
Their indomitable courage, inspiring initiative, and selfless devotion to duty in the great credit upon themselves and the U.N.S.C. Naval Service. They gallantly gave their lives in the service of the U.N.S.C.
Trivia
One of the names in the left plaque says Chen Agu. Possibly a relative of Romeo from Halo 3: ODST.