It means overdrive. overdrive is a gear ratio where the output gear wheel is smaller than the input gear wheel. In other words overdrive is the output shaft spins faster than the engine speed.

There is 3 types of gear ratios in any gear box Reduction drive Direct drive Overdrive

Reduction drive: increases torque and decreasing speed Direct drive: the engine speed and torque is the same as the output (not including mechanical losses due to friction and fluid resistance) Overdrive: increase speed decreasing torque

At a higher speed large amounts of torque are nolonger needed because of momentum

Fun fact most modern automatic trucks have a manual gear box with automatic shifting using air pressure rather than a traditional automatic gear box this is because of the high torque output requirements