OA is your basic wear and tear on the car. Over time components will wear down with use. Brake pads are the easiest way to see this. As time goes on and you use your breaks more and more then pads will eventually wear down and you'll get metal on metal when the pads are completely worn. If you want to liken this to your knee, as time goes on your cartilage will wear down and you'll eventually get a bone on bone when the cartilage is completely worn.
RA is the rust. While there will still be a wear and tear type thing going on, there's also another element at play. Rust will infiltrate a car and eat away at the components. RA is sort of like that, the body has an autoimmune response and will eat away at the cartilage.
Ok, so this is a very rudimentary analogy, but I think it gets the basic point across. Obviously there's a lot more going on with OA and RA than my simple explanation.
Do you find this explanation useful? How do you like to explain injuries to your clients?