Am I correct in thinking that OOW & OOD are basically the same role, but OOW performs at sea, switching every watch while the OOD performs in harbour, switching every day? I ask because I have heard OOW is subordinate to OOD.
In a nutselll yes but I ask to think on your last statement. Why would an OOW be subordinate to the OOD? They are not closed up at the same time and as you pointed out one functions whilst at sea whereas the OOD is your atypical Duty officer when alongside the wall.
Perhaps I heard incorrectly that OOW was subordinate to OOD (it is certainly contrary to the fact that the 2 positions don't occur simultaneously). Perhaps I am thinking of some sort of training position as assistant OOW/OOD (what is the name of such a position?).
Like Ex-Dragoon said, that's the distinction in a nutshell. They are each part of a different type of watch. The OOW is part of the Watch on Deck - which occurs when steaming along, whereas the OOD is part of a duty watch - which happens in harbour, when the ship's not moving anymore. To quote from the MARS II handbook: "The OOW has charge of the ship. This means he is responsible to the CO for the movements and safety of the ship. When on watch, the OOW has command over all ship's personnel with the exception of the CO and XO. His responsibilities include:
safety of the ship and her personnel;
DC state; and
An OOW has many responsibilities but the CO retains command of the ship. To accomodate this CO/OOW relationship, there are a set of orders called Captain's Standing Orders. These outline what the OOW can and cannot do without calling, recommending, informing or asking permission from the CO."
To get to be an OOW, you have to get your ticket - for us reservists, this is at the end of our MARS IV training summer. To be a qualified MARS officer is to have the OOW qualification. The OOD, however, can be either an officer or a senior NCO, as NCRCrow pointed out.
To add a little clarification. It IS possible to have both and OOW and OOD at the same time. In days past when ships went to anchor for extended periods of time, there would be an OOW with OVERALL responsibility for the ship, and an OOD conducting the "duty-watch" daily administration and routine for the ship. It happens very infrequently now, as the OOW generally performs both roles, it was also more common in a larger ship, such as a cruiser (when we had them) or in the AOR (tanker).
Any seagoing officer can (will be expected to) achieve the OOD qualification; in the Reg force the engineer officers and supply officers must get this qualification in addition to their own. For MARS officers, Reg and Reserve, the OOD must be achieved prior to the BWK (if you look at a BWK certificate it says "...is qualified to stand watch in harbour and at sea"). In the NAVRES, the only seagoing officers are MARS.
NCMs can be OOD qualified, but must be CPO2 or above, and then may only be OOD in Esquimalt or Halifax - this is only done in major warships (destroyers, frigates, tanker, etc.); in foreign ports it is always a commissioned officer. In Esquimalt and Halifax for minor warships (the MCDVs), there is a position called "Senior Watchkeeper" (SWK) that performs functions very similar to OOD, this is a PO2 to LCdr position (although in exceptional circumstances a MS can do it); the roles are so close it makes on wonder why the distinction except that an OOD has more legal powers than a SWK.