Green’s Dictionary of Slang

(US prison) a correctional officer.

2010		L. Berney Gutshot Straight [ebook] There was a CO across the room [...] but Shake knew that the guards weren’t paid enough to intervene.
2017		T. Pluck Bad Boy Boogie [ebook] ‘Go on,’ the guard said. Jay couldn’t remember his name, but he was all right, as far as COs went.
2020		D. Winslow ‘Broken’ in Broken 22: One got shanked [...] He bled out before the COs got there.