(O/D) Is overdrive. (Fourth gear)
If you see it lit on your dash, that means that it's off. There is a button on the shifter or below on the console part of the stick with the letters O/D on it.
You want OD on for highway driving but its ok to turn it off for street driving.
Remember, if the O/D light is on, it means it's OFF.
__________________ 1995 Mustang GT 5.0
BBK CAI, Pro-M 75mm MAF, Pro-M Optimizer, BBK 70mm TB, GT-40 Intake, 24# Injectors, AFR 165's, 1.7 Roller rockers, Crane 2040 cam (E-303) BBK Equal Length headers, BBK O/R X-pipe, Flowmaster American Thunder 2 Chambers. FMS 3.73 Gears, Lentech AODE 2800 stall, BBK 255lph Fuel Pump, BBK Fuel Pressure Regulator. MSD 6AL, Steeda UDP's, FMS Aluminum drive shaft, Autologic chip Larocca tune. | 
The o/d light is on in my 1997 Mustang 
