Over drive has to do with the ratio of the gearing. Once it is over a 1:1 ratio, they call it overdrive regardless of whether it is an auto or a standard. The 4th gear in the auto trannys, 5th gear in the GT tranny & the 6th gear in the GTS tranny are all overdriven gears.
Pontiac used to call a locking torque converter overdrive.
Celi automatics have 4 gears L1, L2, D, O/D; but they also have a locking torque converter as well. Excellent setup for the highway. Out autos aren't great for the quarter, but for regular driving & highway driving/passing they work great.