Strange, I would have thought that code completion would be less likely to work since it plays hard and fast with windows, the focus subsystem, and keyboard shortcuts all at the same time!
Does copy/paste work when copy and pasting within the same text area, or does it only not work when you try to copy to/from an external application?
Can you drag-and-drop text to/from RSTA in an Oracle Form? Again, both within the same text area, and to/from an external app? I can't remember whether you need to call textArea.setDragEnabled(true) for this to work, or if RSTA does that for you automatically.
Unfortunately I'm unfamiliar with OC4j, and I'm also not a J2EE developer. If you can provide me with a short example app, and tell me how to set up the environment (assuming it's really simple!) I can try to debug.