Prepare charcoal grill or preheat an ungreased cast-iron skillet over medium heat.
Very lightly brush the corn with oil and set on the grill rack or in the skillet.
Cook, turning every 2-3 minutes, until the kernels are tender and nicely charred, 12-15 minutes total; keep warm.
Meanwhile, in a bowl stir together the fish sauce, water, sugar and salt until it is dissolved.
Heat the 2 tbsp oil in a saucepan over medium heat until very hot but not smoking.
Carefully pour liquid mixture in (it may sputter a bit so watch out); add the green onions and simmer until the sauce begins to thicken, about 30 seconds; remove from heat and cool.