J.T.B. said:
The USN classification of "unrestricted line" descends from the old days of sailing ships, when the officer of the deck had to be constantly alert to changes in the wind and know what to do immediately in response, trimming, adding or taking in sail as necessary to keep course and make the most of the winds. Not to mention the hazards to equipment and the safety of ship itself in case of a sudden storm. Obviously, it took a lot of experience to be trusted in such a position. With modern technology the OOD job has become easier, but still requires knowledge of shiphandling and what to do in all sorts of emergencies.
Serving as OOD underway on USN warships is not limited to URL officers. Indeed, I have heard that it is now common for supply corps officers to be required to stand OOD watches on surface combatants and 'phibs, because officer manning levels have been reduced. They can still not succeed to command the vessel however.
On a Starfleet vessel, I would think that extensive experience would be even less crucial, since 90% of starship travel seems to be going in a straight line through emptiness. It doesn't seem too much of a stretch to think that someone from a completely different field, like a medical officer, could be given enough training to know what to do in the time it takes to call the captain to the bridge.
--Justin