From the Owner's Manual:
Overdrive switch
Each time your vehicle is started, the transmission is automatically “reset” to overdrive ON.
ON: With the engine running and the shift selector in the D (Drive) position, the transmission upshifts into Overdrive as vehicle speed increases.
Overdrive does not engage until the engine has reached operating temperature.
OFF: For driving up and down long slopes where engine braking is necessary push the Overdrive switch once. The O/D OFF indicator light in the instrument panel comes on at this time.
When cruising at a low speed or climbing a gentle slope, you may feel uncomfortable shift shocks as the transmission shifts into and out of Overdrive repeatedly. In this case, depress the Overdrive switch to turn the Overdrive off. The O/D OFF indicator light in the instrument panel comes on at this time.
When driving conditions change, depress the Overdrive switch to turn the Overdrive on.
Remember not to drive at high speeds for extended periods of time with the Overdrive off.
This reduces fuel economy.