I really like both O and Phe, but really you can’t go wrong with any of these!
O is delightfully simple and understated, and Phe I’m admittedly partial too because my mother’s nickname is [name_f]Fe[/name_f]/[name_u]Fee[/name_u] (from [name_f]Felicia[/name_f]).
Edit: looking at the other nicknames and your latest name, I’m still very much in the O camp!
Out of your three choices, I like [name_u]Elia[/name_u] the best, but it seems kinda…off as a nickname? I mean, cutting a four-syllable name (that is also frequently pronounced with three syllables) down to three syllables just doesn’t seem like it would be much shorter or easier to say. I think [name_f]Lia[/name_f] would work even better, and that would be my personal first choice as a nickname for [name_f]Ophelia[/name_f], followed be Fefe/[name_f]Fifi[/name_f] (whichever spelling), which I think is cuter than just Phe. O is kinda cool, but maybe a little too edgy for a little girl? (I can picture a teenager called O, but not a baby or small child.)
BTW, great combo! My daughter [name_f]Cate[/name_f]'s (short for [name_f]Catherine[/name_f]) middle name is [name_f]Ophelia[/name_f]. We often call her [name_u]Cato[/name_u], which might also work for you as [name_u]Kato[/name_u].
[name_f]Ophelia[/name_f] [name_f]Kate[/name_f] is beautiful, and I love how it goes with your other name choices! I’m not a fan of any of the nicknames you listed above though. I prefer for nicknames to sound like names in and of themselves rather than syllables taken out of names- what about [name_f]Lia[/name_f] or [name_f]Ella[/name_f]?
@mega_muffin I love [name_f]Catherine[/name_f] [name_f]Ophelia[/name_f] nn [name_u]Cato[/name_u], so cute and clever