FWIW, I'm not a big fan of OCA theory. Or perhaps a better way to say it is that I think the case for making the entire planet with its next day transport of goods and immediate delivery of services would nominally qualify as a single OCA. Note that Mundells criteria are focused primarily on sovereignty and not economics per se (see the Greek comment in peer comment).

Tariffs and currency manipulation are a sovereign response to prevent efficient markets that would render local labor noncompetitive.

And as I mentioned earlier, a single currency makes such manipulations ineffective and you get Greece. They can't bring their productivity up to the level of Germany or France and their economy suffers for it because all of the capital goes elsewhere.