Why does O/D Off light come on while driving on highway?

Hello, what you are experiencing sounds like there is a malfunction that occasionally occurs with the electronic or hydraulic operation of the transmission. When this happens, the O/D off light comes on, and the system shuts down overdrive. It could also disengage the torque converter -- hard to say without testing and monitoring the operation of various components with a suitable diagnostic tool. Testing is required to see what is happening and what needs to be done to resolve it.