Ops is a job for an Ensign.
It's a waste of resources to use a higher rank.
If Harry wanted to get promoted on Voyager in the DQ, he had to give up Ops and move to a larger department that had room for advancement.
Security?
Security was folded into tactical with Tuvok.
Was that because Tuvok was so awesome that he could wear two hats or was it just that the ship was so small? If it was just because the ship was small, does that mean that Kim is such a dunce that he has to wear half a hat?