Heat the oil in a skillet or wok over medium-high heat. Add the ground chicken, scallions, and cornstarch and cook, stirring constantly, until the chicken is cooked through, and broken up (but do not brown), about 3 to 4 minutes.
Add the water chestnuts, soy sauce, ginger, and oyster sauce and cook for 1-2 minutes. Remove from the heat. Cool slightly.
If you like spice, add some red pepper flakes or a dash of hot chili oil to the chicken mixture.
To serve, generously spoon chicken mixture into the center of each lettuce leaf. Wrap the lettuce around the filling. Top with cilantro leaves and chopped peanuts, if using, then eat like a taco.
