What year was O.N.I First introduced in lore?

Ive been wondering when the office of naval intelligence was actually put into the lore of halo because I always tgought it was first mentioned in Halo 3 first quotation on the mission tsavo highway as “oni recon 111”.

From what I can find, ONI was first mentioned in the novel Halo: First Strike. Released in December of 2003.

Source.

ONI’s been in halo since Fall of Reach. Earliest mention I can find is Page 18.

I think it was in the Fall of Reach

It was introduced before Halo CE came out, in a little novel called The Fall of Reach.