To get QuickTime plug-ins, launch your player. Then go to Edit > Preferences > Quicktime preferences and select Browser Plug-in. Click on the MIME Settings... button.
QuickTime will create the plug-ins only for the types selected in these settings. Therefore, you should select everything to get all the possible plug-ins generated.
Opera 9 will automatically detect the QuickTime plug-ins from the QuickTime plug-ins folder after a restart.
For Opera 8.5x and earlier, copy all the plug-in files over to Opera (to Opera's install folder\Program\Plugins). Some versions of QuickTime (e.g. v7.0.2) will automatically place the plug-ins in Opera's install folder\Plugins, but this is not the correct plug-ins folder and therefore these files should be moved as described above. I now have npqtplugin[-8].dll happily residing in my Opera plug-ins folder. It is also good to copy over any other files (e.g. *.class) from the QuickTime plug-ins folder to Opera's plug-in folder.
Of course, you might still need to adjust the MIME types in Opera to use the appropriate plug-in...