A quick check on an acronym finder - gives two relevant meanings for COS (Chief of Staff & Chief of Station ). Only relevant meaning given for DCOS is Deputy Chief of Staff.
Why would a chief of staff be redacted?
Why would a chief of staff be redacted?
Redacted names could be people that flipped and are cooperating witnesses? (of course the bad guys would then know exactly who is cooperating - which may be the purpose)
Or perhaps those names are still be looked at by Durham and need to be kept out of the public until he’s ready?
Pure guesses.
Probably Chief of Station. Head CIA person at an embassy. Pretty good reason to redact them.