Since SUN jdk 6 update 10, a new generation plug-in is provided.
This is an article with giving lots of information.
To install it under Opera 64 bit, add the [jdk_installation_dir]/jre/lib/amd64/ under the plug-ins path definition (Tools-> preferences-> advanced-> contents-> plug-ins options-> change path-> add). The corresponding library is libnpjp2.so.
You can check your installation with this site applet.