O/D orange-yellow light is on the cluster when it is off.
Now about the function, Platz is an automatic car with four pseudo-gears. I will try to be as non-technical as I can.
When you have overdrive off (light on) , car goes upto 3rd gear. If overdrive is on (light off, default mode) car goes upto 4th gear if required.
If you want a technical explanation let me know.