Hawkeye is right; taken directly from the Opera help:
"The Macromedia Shockwave Flash player is a popular free plug-in for enhancing Web sites. It can be downloaded from Macromedia's Web site.
In order to have the Shockwave Flash player working with Opera, please follow these instructions:
Download the installer program to your hard drive. The installation file is called "flash32.dll".
Double-click it to start the installation and follow the on-screen instructions.
The first thing you'll be asked to do is close all open applications. When you have done so, press the "Next" button to continue with the installation.
The next screen will ask you to select the destination of the plug-in. If you have got Netscape Navigator installed on your system, this browser will show up as a selected destination for the install.
If you do not, the installer program will propose another directory for its installation. In either case, you should do the following:
Press the browsing button, which is named "Other Browser" if you have got Netscape installed, and browse your way through your system's directory structure to your Opera directory.
In the Opera directory, open the folder named "Plugins" and press the "Select" button to confirm the directory and start the installation.
The installer now places the file "NPSWF32.dll" in Opera's plug-ins folder and asks you to press "Continue" to complete the install by going on-line to finish the installation procedure. Do so,
and you should be all clear!
Note! The Shockwave Flash player currently does not support Opera during install, which may lead to error messages appearing on your screen stating that the installer application is not able to locate and launch Netscape.exe. Internal testing at Opera Software has found this to not to be a problem, as the installer will not attempt to launch Netscape.exe until after it has placed the required files in Opera's plug-ins directory, if the above instructions are followed."