Personally, I'd go for Odessa.
Looking at the codenames, a lot of the syllables end in vowels, and those that don't end in softer consenants. Most are under four syllables as well.
With that said, I find Olivia to be out of place with the other names; The hard "v" sets it out like a sore thumb. Ophelia seems too awkward; the other names just smoothly fall off the tongue, while Ophelia almost has a forced sound to it (and perhaps is too region-specific with the "ph" as an "f" sound").
I wouldn't go so far as to leave Linux Mint if either were adopted as the codename, but that's my 2 cents worth. Of course, those also seem to be the two most popular names, so I wouldn't be too surprised if one or the other is adopted.
They say that a three-year old can use Windows 8. I say that's a bit of a regression. When I was three, I could use DOS.