[__rito__]

It is just Horn Please. OK is a historical artefact.

During WW2, due to fuel shortages, Indian trucks often switched to Kerosene.

OK means: "On Kerosene". OK was painted on the back of trucks and other vehicles to warn other drivers to maintain a safe distance because Kerosene is highly flammable.

Due to another meaning of OK, they just kept doing it. [0]

[0]: https://www.fr8.in/blog/why-is-horn-ok-please-written-behind...

[mikestew]

More flammable than gasoline? I’m not buying this one at all.

[detaro]

It says "than diesel" in the link, but I don't think diesel engines would've been common at all at the time. To my knowledge they only became popular post-WW2.