OD will be the HIGHEST GEAR, you have, OD will NOT function at low speeds.
As to whether it will go into 5th from OD, I don't know as each box manufacturer is different, and is likely to be ecu controlled as well, so depends on programming but OD will on all auto's be the highest possible gear, I would switch out OD in town as it creates extra work and wear in the box, and will NOT improve MPG as more slippage will be needed, which is odd as OD is always a TORQUE CONVERTOR LOCKED GEAR, anyway.
tabetha