we do use currencies

Indeed we do. But what examples are there of currencies that are not issued by sovereigns? The point about OCA theory is that it shows little interest in that. Mundell's theorising about it posited, one might think provocatively, that a nation state such as Canada might profitably be divided into two OCAs, East and West. This is not going as far as Hayek, (currency issued by private commercial concerns), but it is a neoliberal ploy to separate currency creation from sovereign nation states.

Whether it is workable or not depends on whether one thinks it possible for a non-sovereign to issue fiat currency. That is the experiment Europe is currently engaged in, and the outcome doesn't look too good. Building a single sovereign out of the multiple sovereigns of the EU should have been the horse before the cart. Instead of which, we have multiple sovereigns deprived of monetary and fiscal policy, and a non-sovereign attempting to enforce an OCA by means of "rules".