You sign articles on an OSV?
There can only be one person in charge of a watch at a time. That person is the OICNW on watch and there’s only one at a time.
When does the captain sleep if he’s up for both watches?
If the master stands a watch then a boat with 12 hour watches has a Master and a Mate and a bunch of extra people. If the Master doesn’t stand a watch a boat with 12 hour watches has a Master, two Mates and a bunch of extra people.