Saving references

DoorAnim

Animator anim = null;

void Start()
{
  // this saves a reference to the Animator
  // way better because you don't have to do an expensive getcomponent each time
  anim = GetComponent<Animator>(); 
}

void Update()
{
  if(Input.GetKeyDown(KeyCode.O))
  {
    anim.Play("DoorOpen"); 
  }
  
  // ...etc
}