XO situation is quite odd in regards to Worf and Kira. Dax was only superior to Worf in "The Way of the Warrior" on the Defiant
. She was in command on the bridge when Sisko returned from Engineering, Worf was at Engineering station. Then, after he was a permanent part of the crew, he was always her superior.
Worf and Kira have a much stranger situation. Kira stayed with Sisko and basically defaulted command to Worf when the ship was damaged in "Starship Down" Worf commanded the Defiant
with Kira aboard and taking his orders in "Paradise Lost" and "Rules of Engagement." In "Tears of the Prophets" when Sisko had to leave the Bridge during battle, Kira stepped in and took over without Worf saying a word.
So strange. To answer the original question, Dax was Sisko's highest ranking Officer during the occupation, so while it may never have been spoken on screen. I do wish that they had given her a Command Uniform for her tenure as Captain, but they do have the easy outs that there was a war on and bigger fish to fry, also, she may have wanted to keep the crew believing that Sisko would return.