I agree with most people here: I think it's a very beautiful name, but the "Oo" is perhaps too fictional-looking (if that makes sense, as if it came from Tolkien). I really do prefer
Una for visual aesthetic. I think it also sounds beautiful spelled either way. And, hey, if
Uma Thurman can make her name work, why not
Una?
The only reservation I have, as Fink stated, is that if you live near Mexico (for the US), as I do (
San Diego), you might have problems, especially if you spell it the preferred "
Una" way. One benefit, it would be perfect for an ONLY child. Hahaha!
Cheers,