Ok, thanks. I studied linguistics for another language years ago, (a language without any 'th'), so it was a guess. Still no answer of what it is or why it's used.
The short of it is that they both were a thing in Western European languages that used to be written in runes. Some of these runes survived for a while as "Latin" letters before being replaced by normal Latin letters. They're only widely used in Icelandic, but very rarely there are English speakers who use oje or both because they feel like it.