Go over to SlackBuilds.org and get the 64bit source download (only if you have 64bit) and download the file, next in a terminal cd to the current place you downloaded the file and un-tar it using the command, tar -xvjf the opera file name.tar.bz2.
After it is un-tarred cd to the file where it is and become root by using the su command, after you are root issue the command ./install.sh after this it will ask questions answer y for yes to these and then that's it Opera will appear in your internet packages.
This is actually my file from slack build;
Once there enter this command;
tar -xvjf opera-10.00.gcc4-qt4.86_64.tar.bz2
so now we need to know the number of the un-packed file and enter the folder,
Once there make sure you are logged in as root and simply issue the command ./install.sh and answer the yes questions.