Microwave chocolate and oil in bowl at 50 percent power, stirring occasionally, until melted and smooth, 2 to 4 minutes. Using toothpick, dip mint ball into melted chocolate to coat, letting excess chocolate drip back into bowl. Return mint ball to plate, remove toothpick, and immediately decorate with sprinkles. Repeat with remaining mint balls and chocolate. Freeze mint balls, uncovered, until chocolate has hardened, about 10 minutes. Store in refrigerator until ready to serve.