Yeah, the jobs got shuffled a bit."Tactical officer" isn't a term used in TOS, anyway, that was more of a TNG thing. In "Arena" "tactical people" are mentioned, but their official positions are vague. The actual fighting of the ship seemed to be handled by the helmsman, with, in later episodes, some kind of "scope" that he used when action was imminent.
In TOS, Sulu did the steering, fired the weapons, and (at least in the Mirror Universe) seemed to be in charge of the Security guys. They apparently took the "steering the ship" part of that job away, merged what was left with Uhura's job, and called that "Tactical". Mix in the "running the sensors" part of Spock's job.
OPS is mostly the job Scotty did from his station on the bridge, but with Chekov's job thrown in occasionally. "Mr. Data, lay in a course for ...." He also sometimes ran the sensors, and answered the phone.
In TOS, helm was the glory job: you got to do everything and look cool doing it. By the time it became CONN, it seemed like it wasn't that special anymore. "Ensign Ricky, would you like to man the CONN station?"
In TOS, "Conn" was the name for the Big Chair. "Inform the Commodore I'll be beaming down immediately. Mr. Spock, you have the Conn."
Forgive me for drifting a bit further away from the topic and towards the rocks as I share my favorite quote from a Trek novel: it is Montgomery Scott, telling someone the six "most frightening" words he ever heard spoken on a starship: "Mister Scott, you have the Conn."