With Opera 11.50+ (fresh install and upgrade thereafter at least), backing up all of Opera's settings is easy.
1. Close down Opera.
2. Copy the profile folder(s) to a safe place. That will backup all your settings and mail and everything.
On linux, the profile folder is located at "~/.opera".
On WinXP?, the roaming and local profile folders are located at:
"C:\Documents and Settings\user\Application Data\Opera\Opera"
"C:\Documents and Settings\user\Local Settings\Application Data\Opera\Opera"
On Vista/Win7?, they're located at:
"C:\Users\user\AppData?\roaming\Opera\Opera"
"C:\Users\user\AppData?\local\Opera\Opera"
If you used a usb install, in the installation folder, it'll be a single folder named "profile" that contains everything.
Then, when you want to use those backups to restore all settings, put copies (so you never put the original backups in use) of the folders you backed up where they belong (replacing existing profile folders that are already there).
That's it. That way, you don't have to do any exporting and importing at all.
Note that for IMAP accounts, the cache of messages in the mail/store folder will be incomplete unless you have "make all messages available offline". So, if you want to use the IMAP cache to backup all your IMAP messages, make sure they're all available offline first. Just note that it's just an IMAP cache (unlike a POP account), and if all messages are removed from the IMAP server, syncing mail will wipe out the cache. That's why it's good to keep a separate copy of just the store folder (for importing into a local POP account and dragging to the IMAP account to upload the messages back to the IMAP server) just in case all messages are lost from the IMAP server.