#define LED 11
char ch;
void setup()
 {
  Serial.begin(9600);
  pinMode(LED, OUTPUT);
  Serial.println("Press O or C:");
 }
void loop()
 {
  if (Serial.available())
  {
     ch = Serial.read();
     if (ch=='O')
      digitalWrite(LED,1);
     if (ch='C')
     digitalWrite(LED,0);
  } 
}