From what I've read (I've never actually had an OBE, at least one that I've confirmed), the difference is that in a lucid dream, you are dreaming, and in an OBE, you're more of in a short coma, so to speak. I'm sure you know what a lucid dream is...Taking Ladusence's explanation sounds good to me. You essentially go inside yourself, and your imagination takes over. You are still connected to your physical body in the sense that you are still affected by external stimuli (wind, sounds, people touching you, etc).

In an OBE, think more of astral projection. Your consciousness separates from your corporeal body, but they are both in the same plane of existence. That is, you aren't affected by external forces acting upon your body (for instance, you can be nudged by someone and arent as likely to snap back into consciousness like you would if you were dreaming), and instead of being able to change the world around you, you can only merely observe the world around you, as it is really happening. You change to a third person perspective, from what I've heard.

I think of it this way...as it's a movie set. For a lucid dream, you are the producer. You can change any aspect. The lighting, the angles at which you view, the characters, the 'script'...

For an OBE, you're only the camera-man. You can change the angles, and see everything as the movie takes it's course, but that's about it.

I hope this helps? Again, this is only what I've learned...It may not be true.