Then in the 4th season, O'Brien trades his one dark pip for this weird little square thing I don't think we've seen on anyone else. Whether that was another promotion, or just some uniform change, no one knows.
It's an embroidered patch featuring three chevrons and two dots. http://images4.wikia.nocookie.net/__...5/Ds9_scpo.png
It corresponds to a master chief in the U.S. Navy.
BTW, is it me or does it seem like the officer to crewman ratio on starships is a lot higher than it is on a real navy ship?
Trek tends to focus on officers, but it could easily be that the majority of crewmembers we see in the background with no discernable rank insignia are enlisted personnel. In that regard, the ratio between officers and enlisted could be the same, except when it comes to away missions.