So, what rank is chief O'Brien?
Well seems he's a nothing. Lower than an Ensign, some sort of Jr. Grade Ensign, or even lower.
So why then, do we see him wearing Lieutenant pips?
This question has long puzzled many Trekkies.
Frankly, the answer is simple
Let me ask another question
"Can a Lieutenant command a "Lieutenant Commander?"
Yes they can.
We've seen this. Remember when Geordi was in command back when he was in red, and that asshole Engineer came to the bridge?
So what does this have to do with anything?
Well, remember that Borg episode where Riker was given a "field promotion" to Captain?
He wore the pips
Then next episode, he was back as a Commander. Almost as if that promotion was not permanent.
And thus, I suspect, our answer.
Chief O'Brien always was a nobody.
For the command structure of the Transporter department to function properly on the enterprise, O'Brien needed to be a Lieutenant
And as such, was given that rank temporarily.