Preheat grill to high. Rub corn and scallions with oil; season with salt and pepper. Grill corn, turning, until charred and lightly cooked, about 6 minutes. Let cool, then cut kernels off cobs (5 cups). Grill scallions until charred, about 2 minutes. Cut into 1-inch pieces. Toss corn, scallions, and mint together. Season with salt and pepper.