Using a second camera and finding it using a tag

I have created an camera which i want to be activated on certain instances when i go to a scene. I start off in a different scene then i change my scene which works perfectly fine. Then i have a camera with a tag i created called OverviewCam. However when i run my program i get the error

NullReferenceException: Object reference not set to an instance of an object

My code for setting the camera is

     Camera OverviewCam;
     OverviewCam = GameObject.FindGameObjectWithTag("OverviewCam").GetComponent<Camera>() as Camera;

Anyone know how to fix this or another way to allow me to change cameras when going to a scene a certain way

OverviewCam = GameObject.FindGameObjectWithTag(“OverviewCam”).GetComponent()

Ok. is this called in either Start() or Awake()??