True, that's why they're 'swing' positions. They can be, and often are, handled by a central security chief. Operations Officer is usually held by the CE, though a COO like O'Brien can be held by an individual. They operate in a gray area of sorts, so not necessarily something that needs a dedicated player, and it can be hard to play without the right understanding. A CNS/Counselor doesn't have a huge need, but they do have shipboard functions. Also a civilian can fulfill that role as well. 

It all kind of depends.