void setup() {
  pinMode(LED_BUILTIN,OUTPUT);
  Serial.begin(9600);
  delay(3000);
  Serial.println("on/off");
}

void loop() {
  if(Serial.available()>0){
    char LEDControl = Serial.read();
    switch(LEDControl){
      case 'o':
        digitalWrite(LED_BUILTIN,HIGH);
        Serial.println("LED is turned on");
        break;
      case 'x':
        digitalWrite(LED_BUILTIN,LOW);
        Serial.println("LED is turned off");
        break;
      default:
        Serial.println("incorrect command!!");
        break;
    }
  }
}