Remove husks and silk from corn.
In bowl, stir together fish sauce, water, sugar, salt and Kampot pepper.
In a saucepan, heat 2 tablespoons of oil until very hot but not smoking. Carefully pour in fish sauce mixture (watch out it will splatter).
Add green onions and simmer until sauce begins to thicken (about a minute). Remove from heat and cool.
Heat grill or ungreased cast-iron skillet over medium heat. Brush corn lightly with olive oil. Cook, turning every few minutes, until corn is tender and slightly charred.
Brush corn with sauce and serve.