O'Brien's rank was so mis-handled over the years
Less "over the years, and more "initially."
"Tactical officer" is a position rather than a rank
Referring to O'Brien as a tactical specialist
probably would have been better.
Strictly speaking, was Maxwell complimenting O'Brien on being a knowledgeable tactician, or a talented gunner?
It pretty much goes without saying that O'Brien would have known the systems
inside and out.