Is using Action.Invoke considered best practice?

The two are equivalent, the compiler converts OnAdd("It Happened"); into OnAdd.Invoke("It Happened"); for you.

I guess it’s a matter of preference, however I personally prefer the terser form.

As an aside, it is generally preferable to take a local copy of a class level delegate before invoking it to avoid a race condition whereby OnAdd is not null at the time that it is checked, but is at the time that it is invoked:

private void SomethingHappened()
{
  Action<string> local = OnAdd;
  if (local != null)
  {
    local("It Happened");
  }
}