o-ou hasn't helped us much, it's only got 9 questions and 0 followers. It has no tag wiki.
I'd vote against it, although I don't feel strongly about the matter.
Sure, it's only got 9 questions. So it's not super popular, but that also means it's not doing that much harm. Nine is at least enough that it doesn't feel like a waste to me when I click on the link to see the tagged questions.
In fact, I think I've used that tag before to quickly find other questions about this subject. It's a bit harder to use search for this, as a search for "o ou" has various irrelevant results about the diphthong [ou] or truncated words.
I don't think small, rarely used tags like this are anywhere near as much trouble as vague, common tags like "usage" or "grammar."