Pleased to hear my posts are helping someone, I mostly respond to queries on Ancestry.com's Military boards and it's sometimes disheartening that many people can't even take the time to say a simple thank you for what is sometimes several hours of searching on their behalf.
Now pre-ordering documents. I mentioned them in my original posting but you may not be familiar with searching the NA Catalogue.
On red menu bar select "Search the Archives" > the catalogue. Click on large red words "Search the Catalogue" Enter the words - Okehampton AND Oswestry - [or you could enter - Prisoners AND Okehampton or - prisoners AND Oswestry] in this case no other detail is needed but normally one would enter a range of dates and because Prisoners of War at the time came under the Admiralty the Code ADM
As you will see there are only two hits ADM103/594 and ADM103/595 "Parole prisoners at Okehampton and Oswestry" and dates covered.
In both cases that is the lowest level the "series" has been catalogued [indexed] to so I assume that is what you need to pre-order although personally I would email the NA to confirm that there is no more detailed breakdown.
Click on either title, then the "Full Details" tab [then scroll down a little] and it reveals the full description of document series ADM103 as: Records of Medical and Prisoner of War Departments - Navy Board and predecessors: Prisoner of War Department and predecessors: Registers of Prisoners of War