Solve me the riddle. I think he was commander at one point on DS9. If so how did you determain captain in that pic? (I miht be wrong with the comander, but I still think I have some dialogs ringing in my head saying cmd. Sisco).
I can go on a bit too much with this first section (and others have spent a lot of time thinking about and discussing it to try to make sense of certain inconsistencies), so I will try to keep it brief. In universe, at the time that series will take place SF uses collar pips as rank insignia for officers, so that would be the main method of identification of his rank. It is a bit hard to see in the picture, and sometimes they mess things like that get messed up.
The inconsistencies start when you look at enlisted personnel. O'Brien is clearly shown with a solid pip at times, yet is still called "Chief" in TNG, however this can possibly be explained away if "Chief" is part of his title or position, not his rank (I think he was called "Lieutenant" early on, but not given a name until later). That works until Worf's father (the human that raised him, not the corpse) speaks to and of O'Brien as being enlisted, if not earlier. Eventually they fixed it on DS9 by using distinctive enlisted rank insignia. This may help:
http://memory-alpha.wikia.com/wiki/Starfleet_ranks#The_24th_century
The easiest way to know his rank is familiarity with the show. He started shaving his head and not shaving all of his face around the time he was promoted (I think facial hair came first, and I heard the reason they didn't let him shave his head at first was that they didn't want it to seem like all the top dogs are bald men or something). Next came the uniform change to the black and grey with the colored undershirt, so he is at least a captain in that uniform, unless he was demoted.
Damn you! I am still gonna post this.
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He is running a station and not a ship, how could he be a captain?
What happens when their are two captains on a ship?
I was always amused that a Major (Kira Nerys) was taking orders from a Captain, since she had been promoted from Captain to Major. (Yes, I understand how joint operations work, I just get a kick out of naval rank. It is also completely logical for her to be second in command and XO to someone that would be the equivalent to a Lieutenant Colonel (when he showed up), which would be her next stop. Did she ever get promoted on the show?)