If you try to use barcodes in Oracle Reports there is a nice tutorial in the docu that can be found here. Basically it works by using a simple (and very old) java-bean (oraclebarcode.jar) that generates that you pass string and it generates the barcode (a graphic) that is displayed in the report.
In one step they tell you to set your REPORTS_CLASSPATH variable, so that Reports can find the java-class. It works fine on the local Reportsserver. What they don't mention, and what every time i am using it takes me hours is that you have to configure your "big" Reportsserver also, and that does not mean to simple set your REPORTS_CLASSPATH, no you have to change the config-file to include the classpath in every engine that is defined there. For example: