USA might not be an OCA (I think it is silly to suppose that an OCA could not even exist - we do use currencies. Everyone having his own does not work, having a single one would likely not work either - so there must be some reasonable point in between).

But it is clear that the Eurozone is much less so, and I don't see how this would fail to be relevant.
My take is that the biggest problem is the lack of fiscal transfer, and (but they are linked) the political acrimony between countries.
In the US, despite the history of a civil war, a massive political chasm between the southern states and the rest, pretty much nobody (certainly from the states providing net support) is proposing ending major fiscal transfers. Contras that with the attitude of Germany, and much of the EPP, towards the periphery. I don't see that you could turn that into an OCA even with a single language and totally free movement of workers.

Earth provides enough to satisfy every man's need, but not every man's greed. Gandhi