To simpify things,
1. When you engage OD button, OD light appear on your panel with label "OD OFF"
2. This is to remind you to turn it OFF
3. Press OD button again, OD light will disappear on your panel
4. This tell you that you car is back to nromal driving mode.
Hope this help. 
Some changes :
1. When you engage OD button, OD light appear on your panel with label "OD OFF"
- This is to tell you that the gear ratio is lock to highest Gear 3 (Gear 1 =>2=>3).
2. Press OD button again, OD light will disappear on your panel
- This tell you that you car is back to normal driving mode (Gear 1 =>2=>3=>4)
Something extra (
http://en.wikipedia.org/wiki/Overdrive_(mechanics) ):
Overdrive can refer to two different things. An overdrive is a device which was commonly used in automobiles to allow the choice of an extra-high overall gear ratio for high speed cruising, thus saving fuel, at the cost of less torque.
It also refers to a combination of gearing inside a transmission or transaxle which results in the output speed being greater than the input speed. In the latter case, "overdrive" does not refer to a physically separate identifiable part/assembly.