#define LED 11
char ch; 

void setup() {
Serial.begin(9600);
pinMode(LED, OUTPUT);
Serial.println("Press C or O:");
}

void loop() {
if (Serial.available())
{
ch = Serial.read();
if (ch == 'C')
digitalWrite(LED,1);
if (ch == 'O')
digitalWrite(LED,0);
}
}