Normally, but not always Navigation is a separate department, even though its officers are seen as preparing for the command branch.
Auxiliary craft units and fighter squadrons call their XO Deputy Commander (DCAG), but duties normally remain the same.
Jean Luc Picard is one of the very rare exceptions of not serving as an XO prior to getting his own command. For those not already in the Command Branch, they must transfer into it prior, even if they are serving as a department head in another branch. Exceptions were made in the 23rd century, but no longer.