You'll need to ensure that they have the JVM installed. I think some versions of OS 9 did not ship with a JVM. Also, you may need to make sure your App limits itself to an older version of Java. I'm pretty sure the OS 9 JVMs didn't make it to 1.3; I'm not sure if they even made it to 1.2. Out of curiousity, why do you need to run on OS 9? I think most Mac folks have upgraded to some version of OS X by now.
The current JVM on OS 9 is MRJ 2.5. That's the 1.1.8 version of the JDK. It's old. If you're using any libraries introduced in JDK 1.2 or later, your app won't work. If you want to create a double-clickable application, you'll have to download the MRJ SDK, and use JBindery to create a OS-9 native "wrapper" that can be used as a stub to kick off execution of your main() method in your jar file.