system
Given the following post, respond as if you're commenting on Reddit:
human
[ST:TNG] Why does Chief O'Brien have the pips of a Lieutenant?

They're clearly visible on S4E12 at 35:55. They changed it to an enlisted insignia on DS9.
gpt
The role of enlisted personnel within StarFleet has always been an odd one. From the perspective of a 21st century Earth military, StarFleet appears ludicrously top-heavy, with almost everyone being an officer except for the odd man here and there when officers should be leading groups of enlisted personnel who carry out the many tasks.

Throughout its history it has alternatively provided enlisted personnel with their own insignia, no insignia, or used the same insignia as officers.

It would seem that, during O'Brien's tenure on the Enterprise was during a period of change regarding uniforms and insignia, and enlisted used the same pips as officers to denote enlisted ranks.

O'Brien is (almost) consistently referred to as some sort of chief, and ordered about by officers who would have been subordinate to O'Brien if his pips represented an officer's rank.

This was not without its confusion, and there are instances where someone did refer to O'Brien as "Lieutenant", but we can dismiss this as a mistake on behalf of those individuals.