The D with the O around it is overdrive. It's the next grear ratio above 3rd. Overdrive on an A4LD is 4th gear. Overdrive on a 5R55E would be 5th gear (virtual 5th). The PCM combines ratios to make 5th gear. Newer transmissions have 6th, and 7th gear. CVTs (continuously variable transmissions) are totally different. Each gear setting is not a specific ratio, since the drive is cone shaped. It would be categorized as a minimum, and maximum ratio with infinite gear setting in between those 2 numbers.