serialEvent.c

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  pinMode(13, OUTPUT);
}

void loop() {
  
}

void serialEvent(void) 
{
  char data;
  data = Serial.read();
  if(data == 'o') {
    digitalWrite(13, 1);
  }
  else if(data == 'x') {
    digitalWrite(13, 0); 
  }
}