How can I cast object to it original type?

I have a method that can return many types of objects (that's why it return just object).

How can I cast it to it's original type? (The question is about casting to it's original type, when I don't know it's type on run time. It is not about casting when I know it's type; that's why GetAnyObject method is black box)

private void func()
{
    var obj = GetAnyObject();
    callFunc(obj);
}
private void callFunc(object o)
{
    var a = o.GetType();
    ...
}