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> The Rails apps required a few small config changes which I could get from the documentation

What Rails configuration would be pertinent to Oauth? Perhaps you mean the application was already using some kind of third-party authentication library (e.g. Devise) that supported Oauth? If that's the case, didn't you just luck out that the developers happened to choose a library that 1. Was still being maintained. 2. Had already added Oauth support?

Most of the Rails apps I have encountered in my career (especially those predating Oauth popularity) used hand-rolled authentication, each different to the next, so it seems that you could have just as easily fell into the same trap you saw elsewhere. It is not clear how Rails saved you here.