Hi Par!
You can't. But you could always store it as a reference and pass it to the callback as a lambda:
public void someMethod(Transform transform) {
Tween t = transform.DOScale(Vector3.zero, 1);
t.OnComplete(()=> doSomethingAfterTween(t));
}
public void doSomethingAfterTween(Tween t) {
Tween theTween = t;
}