I think its to do with the system of asking someone to honk the horn in case they want to overtake. \"Horn Please\". Unlike the west where honking the horn is a way of showing displeasure, we in India believe in making liberal use of the horn when we are bored, when we are happy, when we are frustrated, when there is a pretty girl in front driving her scooty

(get the drift??). Anyways... the \"OK\" comes in where the driver in the front wants to signal the vehicle behind its safe/ok to overtake. Think there was supposed to be some light over the \"OK\" (which was centered between the \'horn\' and \'please\' for more \"fashionable design\") .. which when switched on would mean... GO AHEAD!! Though this practice has been long \"extinct\" and the use of hand signals (and i am not talking about displaying the length of ur middle finger) has taken over, guess these truck drivers find it extremely difficult to get rid of age old customs.
