Does saying “good Lord’ or “Oh my Lord” count as using the Lord’s name in vain?
I believe it does. I avoid saying such things and have taught my children not to say them.
What does it mean “to take the Lord’s name in vain”, anyway? Well, what does it mean “to take the Lord’s name”?
When we are baptized, we take upon ourselves the name of Jesus Christ. As I understand it, that means that we are supposed to act as Jesus would act, do what Jesus would do, in every situation. In other words, we are supposed to take Jesus’ place. We are supposed to be like Jesus. People should be able to look at us and say, “Oh, look, there is a disciple of Jesus”, just as you might look at Sister Jones’ daughter and say, “Oh, look, there is a Jones girl”. We should be carrying the image of Christ with us.
So then, what would it mean to take Christ’s name in vain? It would mean that we take the covenant of baptism, wherein we take Christ’s name to ourselves, but then we don’t actually do the Christlike thing. We “talk the talk” but we don’t “walk the walk”. We claim to be “Christian”, but those looking at us and our actions and speech can’t tell that we are Christ’s, because we don’t act like Christ.
In this larger sense — which I believe is the fuller, truer sense of the commandment — anything we do that is unChristlike is “taking the Lord’s name in vain”.
That phrase has traditionally been understood to mean that we use a name (or title) of God in a profane, unholy sense. This certainly is an unChristlike thing to do; we should never profane God’s name or title, using it as a swear word or expression of surprise or displeasure. In this sense, I believe that the examples you mention are indeed a vain, vulgar use of the name of God, which should be used only in reverence.