DH and I are expecting our fourth baby in October.
I've never heard of Othelia but I do like Ophelia. How about Tilda with nn Tilly?
It does have a pretty sound, and I like the family connection. But I think she would be forever correcting people's pronunciation and spelling. Not only is it unusual, the "th" is a very soft sound that would be easily misheard. Some similar-sounding options:
Congrats on baby #4! Othelia is pretty, but everyone will mistake it for the name Ophelia. You might consider using Ottilie(which I love), instead. It is pronounced "ah-TIL-lee", so you get Tillie, easily. It is a cool name that is a bit different, but not weird.