Can I teach the Scrivener dictionary to “learn” an old English phrase? I don’t want to have it learn the individual words because then it may ignore the misspelling of similar words. Is there a way to have it learn an entire phrase?

There isn’t a way of doing that. Just to be clear Scrivener uses the Mac’s spell check engine, so it will act the same as it does in Safari, Mail, etc., and we have no control over how it works.

This is a significant problem for this particular piece of software and should be addressed in a future update. The reason is that this software is aimed at writers who are much more likely to be using unusual words, eccentric spellings, and so forth, than the average word processing user. For example, I am writing a novel with a character named “Odo.” Every time I type in his name, it autocorrects to “do” and there is no way to turn this off.

Yes there is.

That’s what Scrivener > Preferences > Corrections is about.

As is System Text Preferences.

As is the idea of telling Scrivener to learn Odo.