I just bought a 2002 RX 300 and this "O/D OFF" light come on randomly??? What does it mean.

 

"O/D" light on means overdrive is off. There's a button on the end of the shift handle that allows you to take your RX out of overdrive when towing a large boat or heavy trailer to keep from overheating the transmission.
The position of the button makes it fairly easy to accidentally engage or disengage overdrive.
There may be other reasons for your problem that I am not aware of.

 

Welcome to Club Lexus!

Overdrive is essentially 4th gear. When the "O/D OFF" light is on, it means the highest gear available will be 3rd gear, it will NOT go into 4th.

You want to keep Overdrive on, in normal driving conditions. It improves fuel mileage, and keeps the torque converter locked.

You can turn overdrive off coming down steep hills, so that you don't overheat your brakes by continuously applying pressure on the brakes.

 

If you are not forcing the overdrive ON and OFF by the switch [99 RX has it on the shift lever next to the gear lock release (thumb operated button on the shift lever), then there is some thing going on with your transmission. Have it checked.

Salim