Is it correct to say, "Amen, O Lord" instead of "O Allah, Amen"?

The wordings for "Ameen" are numerous, and the matter is broad. So, if a person says: "Allahumma Ameen" (O Allah, grant it), or "Ameen, Ya Rabb" (Ameen, O Lord), there is no harm in it, because it fulfills the purpose, which is to ask Allah to answer the supplication, whether the "Ameen" is for one's own supplication or someone else's. We do not know of any scholar who said that the wording "Allahumma Ameen" is incorrect. An exception to this is when a person completes Al-Fatiha in prayer; the Sunnah is to restrict oneself to saying "Ameen" only, due to the saying of the Messenger of Allah, peace and blessings be upon him: "When you pray, straighten your rows, then let one of you lead you. When he says 'Allahu Akbar', then say 'Allahu Akbar'. And when he says: 'Ghairil Maghdoobi 'alaihim walad-daalleen' (not of those who have incurred Your wrath, nor of those who are astray), then say 'Ameen', Allah will answer you."