It's a bit amusing that TOS was never an ensemble show, yet that's where the tradition of summoning all the regulars and almost-regulars to a single non-bridge room and occasionally letting others besides the three stars contribute was born. But TOS did have actual briefings, top-down, to large numbers of people; the TOS movies just had chit-chats between two or three people who were socializing first, sharing information hierarchically second. Except for the TMP Rec Deck scene.
"Kirk's Office" is probably a somewhat misleading name for what appeared to be his personal quarters, complete with a bed, a dining corner and whatnot. Kirk in TOS often talked with his two co-stars in the privacy of his cabin, too. If he had a separate office, he didn't appear to use it for anything much. Or should we decide that the room on Deck 12 and the identical room on Deck 3 (or, in "Journey to Babel", Deck 5) were different in function, one being where he lived and the other where he did business?
Since no other Trek skipper really had an Office, either, we might consider such a thing conceptually outdated in the 23rd-24th centuries. But the Ready Room of TNG and VOY serves in that de facto role, and its absence in TOS or the TOS movies thus does stand out. As for the new movies, we can still wait and see.
Timo Saloniemi