What is the ruling on saying "May Allah bless you, O Lord" or "May Allah make you happy, O Lord" in response to a question about one's condition and news?

There is no harm in the phrase "Ya Rabb" (O Lord) at the end of speech, for its meaning is "O Lord, answer [my prayer]," and the intention behind it is to supplicate Allah the Almighty. It does not mean that the one being addressed is a lord to you.