Create a label in the "Labels" access point in the mail panel. Right-click on that label and goto "properties". In the options, check "hide these messages from other views". Then, apply that label to messages you want to archive (or drag those messages/selection to that label). Then, those messages won't show in other views unless those views have "Show hidden" checked (click the "settings for this view" icon on the mailing list toolbar for each view to see).
That hides them, which is close to archiving.
You can do the same for IMAP messages. But, before you apply a label to them, you might want to move them to a different IMAP folder. It all depends on if you use the IMAP folder views in Opera or just the "All Messages" views or both, and whether or not you use other IMAP clients and webmail etc.
Archiving in IMAP though usually just means to move messages to an IMAP folder that's designated for archived messages. In Opera, you do this by dragging the message (or selection of messages) from the IMAP folder list it's in to the IMAP folder view you want to place it in.
Now, for IMAP, if you want to remove the messages from the server and keep it in Opera, there's no direct support for that. But, there's a workaround. Ctrl + s a messages and save it as an mbs file (or create a label, put messages in it, right-click the label and export the label as an mbs file). Then, use "Menu -> settings -> import and export -&^gt; import mail-> import generic mbox" to import into a new account (which you can name what you want in its properties). Then, you can delete the originals that are in the IMAP account.
The next time you import archived messages, you'd import into the existing archive account if you want to.
Also, Gmail IMAP does something different/special for archiving.