"The operations officer was a Starfleet position within the operations division which was common in the 24th century.
On board starships, the operations position (also known simply as "ops") evolved from older 23rd century positions. The bulk of the duties held by the helm and navigation positions were combined into the conn position. Other functions of the helm panel, such as internal systems control, became the purview of ops, as have some communications and sensor system usages. Furthermore, ops personnel coordinate the scheduling of resources, hardware and system usage for an entire starship, outpost or space station.
The ops position is typically manned by a member of the command staff of a starship, regardless of seniority. On the USS Enterprise-D, Operations Officer Data (who doubled as second officer) was the most senior member of the ship's staff save the captain and XO, but on USS Voyager, Operations Officer Harry Kim was the most junior bridge officer. As analytical operations relied upon knowledge of science and engineering disciplines, operations managers sometimes performed the additional duties of a science officer, as exemplified by both Lieutenant Commander Data and Ensign Harry Kim."
- source - http://memory-alpha.org/en/wiki/Operations_officer
Chekov's role evolved into the position now known as OPS, though he was considered a Navigator at the time (Sulu being the helmsmen).
OPS is a position which first appeared in TNG as such (or at least I think so). The idea is that Enterprise D is a big ship with not only different departments, but many external science and other teams attached to the Enterprise, but not necessarily part of the crew.
So there has to be someone who is going to coordinate all these different functions, allocate resources, repairs and all other operations both related to ship functions and the activities of these attached personnel.
For example if there is a science team onboard from some institute doing a research on something, they might need to use the primary sensor array to gather data on a nearby sun. OPS is the one that has to deal with such needs, together with everyday ship operations.
there have always been science officers - Blue
and there have always been engineering officers - Gold
and there have always been tactical officers - Red
and command is also now Red
Data was an engineering officer as such had a gold middle to his jump suit during TNG then a gold collor in the movies
and kim was exactly the same
science officers were officers who were trained for scientific theory and as such that includes biology ie medical
so crusher was the leading science officer on picards enterprise
then on voyager it was the doctor untill later when they gave the theoretical sciences position to seven
and if this isnt what your asking about im not sure what your question means there have always been the three different Positions
as for why the first officer has been tactical in every show after TOS thats just the way they worked the officers carreer