Some sites (applications) require java to start working properly. For example some KVM switch will give you access to work with system via java applet. Opera doesn't have built in applet, but you can point Opera where to take it from.

First you should install java as usualon your linux system. You can select any java package (sun, ibm, bea etc). For example, on RHEL or any other rpm-based linux distro you cat type

Find where jvm library libjvm.so file is

and export the path of the directory where this file is located via LD_LIBRARY_PATH variable (better to put in ~/.bashrc)

After this just restart Opera browser. This will give you possibility to use Opera on LInux with sites that require java.