why go out of the way to make his character a non-commissioned officer - what was the thinking here
Here's my thought:
To make him more relatable. O'Brien was fairly rare character on Star Trek, he was the Everyman. Sure, most folks may think of themselves as the Captain, or the doctor, or some super smart, strong, attractive character...in the end most of us are O'Brien.
On a station and a starship full of statesmen, androids, shapeshifters, centuries old symbiotic aliens, genetically engineered humans, alien terrorists, blind geniuses etc. he was just a guy doing his job well and had been doing it for years.
He wasn't top of his class. He had prejudices. He had a wife and kids, and would have a nice drink after work with his buddy. At work, the sleeves are rolled up and he was one of the guys.
And another way of showing that is making him a non-com. He's not the manager, he's the foreman. Star Trek plays fast and loose with its titles, and doesn't exactly sync up with present-day military, so it really only needs to be measured against itself.
As a non-com in command situations he subservient to the lowest Ensign (established when he told this to Ensign Nog, fresh from the academy), but his position makes him top dog in Operations and Engineering. I can't think of any examples, but I'm sure he's been in charge of ensigns and lieutenants when it falls under that umbrella.
Just my take.