In a comment to this answer it was mentioned that おはようございます can be used in a variety of situations other than in the morning.
To me it only makes sense to say "good morning" when it is actually morning in some sense. Are there truly situations in which おはようございます is appropriate, but "good morning" would not be?
おはようございます is often used as a start greeting in work situations, whatever time it may be, as opposed to the end greeting おつかれさまです.
I know it's used in the TV and entertainment industry (it was the question of a quizz), and I've heard teachers use it to welcome their pupils as late as 19:30.
おはよう or おはようございます is used when the time is considered as the beginning of a day in some sense. For example, if the addressee has just waken up, it is appropriate to use おはよう(ございます) even if it is not in the morning. On the other hand, if you stayed very late at workplace until 1 o’clock in the morning and met a colleague who was likely to have done the same thing as you, you would not say おはようございます, because it is considered to be the continuation of the day before rather than the very early period of a day.
Apparently お早うございます & お休みなさい are used among family members but not こんにちは & こんばんは. At some work places people regard other workers as similar to family members and consequently avoid using expressions that are used with non-family members. お早うございます is used in place of こんにちは & こんばんは and hence used at any hour of the day or night.
(This comes from Nihongo notes 10 (p. 48), published by the Japan Times, written by Osamu & Nobuko Mizutani. I have talked about this with people but not really experienced it although as foreigners we are perhaps less likely to be welcomed into the "family circle" so quickly with a less than usual aisatsu.)
But consider that "good morning" is but a cross-language approximation.
Literally, おはようございます means "you are early" (honorific お usually used when addressing the second-person as opposed to the first-person). So I expect it to be used also in situations where earliness is being described.
I was taught that おはようございます when not used as a morning greeting, is basically saying 'hello, this is the first time seeing/meeting you today'. This is something that can be difficult to understand to anyone new to the language.
Let me give an example: It's 10PM and a group of 'friends' enter a bar, and the bar tender, whom they are close to, welcomes them in and they all say おはよう to each other. This would be strictly translated as hello/hi/hey. Now understanding the greeting of おはよう/おはようございます as 'you are early', is more like, "Hello, I am seeing you for the first time today".
It can be temping as a speaker of English to take this phrase only as good morning. I have traveled to Japan and I remember I was walking down a busy street around 9PM to hear a man say おはようございます to someone. I was very confused at the time, but after learning more about the use of おはようございます, I have learned that it is very common in Japan to use おはようございます if someone is meeting each other for the first time that day.
