yes, i know what you are saying. But why was 'yn ol' marked as being wrong? Obviously, nol and yn ol are both correct but duolingo marked yn ol as being incorrect.
There seemed to have been a problem with an earlier edit to the database to allow both yn ôl and nôl, but it has now been fixed. .
yn ol means back to. nol means to fetch. according to Geiriadur yr Academi a Geiriadur Prifysgol Cymru (can't do the diacritics here)
As EllisV says, it is also a contraction of yn ôl - which one is meant can depend on the context. In writing it is usually, but not always, yn ôl, but both tend to sound the same when spoken.
Later in the course you will meet another meaning of yn ôl, which is 'according to'.
I had read about the meaning of 'back' in the Ap Geiriaduron. But hovering over the word in Duolingo, only "to fetch" is mentioned.
It might be worth adding this meaning to the hover notes.