void setup() {
  // put your setup code here, to run once:
  pinMode(2, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  // put your main code here, to run repeatedly:
  if (Serial.available() > 0) {
    char inpt = Serial.read();
    switch(inpt) {
      case 'O':
        digitalWrite(2, HIGH);
        break;
      case 'F':
        digitalWrite(2, LOW);
        break;
    }
  }
}