Though origin of this phrase is unknown and there is no official significance with respect to Indian traffic regulations. Following are the few theories behind it:
In the early days, most of the trucks in India were manufactured by the TATA Group. During these times, Tata Oil Mills Ltd. Co (TOMCO) came up with a new brand of detergent called “OK”. And in order to market their detergent they used the motor medium very effectively by painting OK.
In the Second World War where trucks were often run on petrol when there was a shortage of diesel. Petrol, 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 Petrol”.