I came across an article by the Points Guy on passport requirements, and it provided some information about which Caribbean ports are not participating in the WHTI. It's not complete, but the key paragraph says:
"Yes. Even if you're on a closed-loop sailing, certain islands in the Caribbean – Guadeloupe, Martinique, St. Barts, Barbados and others -- require a passport for entry. It’s your responsibility (not your cruise line’s) to learn the requirements of each of the destinations you're visiting before you go."
St Bart's is on our closed loop itinerary in February, so a passport is required with 6-month validity upon departure. (Neither Barbados nor Guadaloupe nor Martinique have the 6- month's requirement.)
Since O sails to many of the the smaller ports in the Eastern Caribbean, and since the itineraries are often quite different from itinerary, to me, it makes sense that O should have a blanket 6-month policy for all their cruises. It would be a nightmare to fashion different requirements on a cruise by cruise basis, and a risk for an accidental screwup which could negatively impact passengers on the cruises. It's safer with the blanket requirement, especially since there can be mid-cruise port changes.
I don't know for sure, but I would bet that the big lines such as NCL avoid those ports that have the passport requirement (6 months or not).
And it's never a good idea to think that a major corporation could be flexible on it's requirements because some other company has different requirements since one exception could open the flood gates of troubles.
Different Oars for Different Ships.
Here's the Point's Guy's article:
https://thepointsguy.com/guide/need-passport-cruise/