Socha he kya kabhi . . .
The popular 'Horn OK Please' seen on almost all trucks in India bears its roots in the second world war where the trucks were run on kerosene engines.Kerosene being highly unstable in nature would cause the trucks to explode at the slightest accident.Hence a warning would be painted on the back saying 'Horn Please,On Kerosene'.Gradually this became a norm and is still seen on most trucks even today.
The origin of this phrase is unknown. Also, there is no official significance with respect to Indian traffic regulations. No rules in India mandate or suggest the use of such a 'slogan' on a vehicle. Still tens of thousands of vehicles are decorated with the phrase Horn OK please. This is also reflection that majority of countrymen (for being politically correct I will use countrymen of this generation) are more of effort driven than result oriented. :) :D
As per typical Indian culture a request is also considered as an umbrage so to make the request more polite they have added "OK" (which literally means I am OK you are OK or okey-dokey ).
P.S. - majority of this content was pulled out of wiki. Next time I promise, will be my own.