There's really nothing you can do here. The views under "All Messages" are hard-sorted the way they are. Even if you try to change that in index.ini in the mail folder, Opera will revert your changes on you.
IMAP folder views and label views etc. are sorted alphabetically (simple ascii comparison) in ascending order.
For labels though, you can prefix their names with an "_" for example to get them to show up above the "a" labels for example.