For POP accounts, under the firstname.lastname@example.org access point for the account in the mail panel, you will only have "Inbox" and "Sent" views. You cannot create other views there. There's also no "Trash" view. There's just the global "Trash" view under the "All Messages" access point.
To create custom views, you have to create a label under the "Labels" access point. The labels you create cannot go anywhere else. Like the "All Messages" views, label views show messages that match from all accounts.
For labels, messages can be manually added to them or automatically added to them via the label rules.
Now, even though "All Messages" views and "Labels" views are global (showing matching messages from all accounts), you can right-click in the mail panel, goto "show messages from" and select a certain account. That will cause only matching messages from that account to show in the views. Note that "show messages from" resets to "all" accounts on startup though.
Opera doesn't support folders at all. But, if you right-click on a label, goto "properties", then "options" and set "hide these messages from other views", when a message is in that label, it won't show in other views if the other views" have "show hidden" unchecked (see the "settings for this view" button on the toolbar above the message list for each view). This allows you to make a label act a lot like a folder. But, you still can't put the label under the email@example.com access point.
So, as you can see, with the "Show messages from" option, you don't need to use the firstname.lastname@example.org access points in the mail panel at all. You can click the wrench icon on the mail panel toolbar and hide them. Then, you just use the "All Messages" and "Labels" access points for example, along with "Show messages from".
Now, in the "Labels" access point, you can create Labels and visually nest them. You can also use rules like (To contains email@example.com) to automatically grab all received messages from a certain account. You could also name the top label named firstname.lastname@example.org and put other labels for that account inside it. For the labels inside it, you could use the "match messages in" option to point to the parent label and then add rules to further filter. Or, you can forget rules altogether and just manually add messages from only a certain account to those labels for that account. It's up to you.
Note though that in Opera, you should only create labels when you really have to. In Opera, the main way of finding messages is to just type in the search field.
See http://operawiki.info/HowM2ViewsWork for more info.