To add toolbar buttons for 'View Text Only' and 'View HTML' in the email window, open the 'toolbar.ini' file located in the Opera directory and look for the '[Mail Standard Toolbar]' section then add these lines to the bottom of it:
ButtonX, "Text"="Set mail display type, 0, , , "View""
ButtonX, "HTML"="Set mail display type, 1, , , "View""
Change the 'X' to the next available number from where you added the lines.