Add the oil, fish sauce, soy sauce, sugar and water and stir until the sugar is dissolved. Pour the marinade onto the chicken and mix well. Let marinate overnight, or at least 2 hours; stirring halfway in between if you can to make sure marinade is well distributed.
2 tablespoon neutral oil, 1 tablespoon fish sauce, 1 tablespoon soy sauce, 1 tablespoon sugar, 2 tablespoon water, 1 lb chicken thighs