There is not a lot of meaning in this expression. People don't pay a lot of attention to it, grammatically. Literally, It is an exclaimation. It directs the statement to non existant a third party. it is followed by a statement that is general, that is for the attention of all listeners. Generally it signifies excitement, but it can be said with sadness or despair. When spoken, this tone will be exageratted. When written, you just take the context of the following statement, to indicate whether it is positive or negative.

1. This is the same as in Japanese. すごいんじゃない？So this one shouldn't be a problem. 2. These are exclamations in the first person, and are quite common in English. Examples: Am I tired! Am I exhausted! You won't see this in the second or third person.

Well, as said before, "Oh boy!" doesn't mean a lot. I just makes the statement seem more important. I can see that you don't speak English very well, so I won't be using any big words, but I'll try to explain as good as possible. "Oh boy" is kind of similar to "God" or "Oh my God", but not quite. See, "Oh boy" is not as strong exclamation as "God" or "Oh my God", but serves similar purpose. When you say "Boy, I'm tired", the "Boy" part can be easily removed and the meaning would stay the same, but again, not quite. There is an equivalent to "Oh boy" and it's "Oh man", so you could say "Man, I'm tired", which would be the same as "Boy, I'm tired". The translation would be "Oh, I'm so tired". If you say "God, I'm tired", the "God" part makes it really strong, so it'll translate as "Oh, I'm really tired a lot". If you use "Oh my God" instead of just "God", it will be a really strong exclamation, as in "I'm so tired I'm going to sleep for a whole week!". I really hope that this helped is any way. Good luck learning English! Sayonara! :D

1 is sort of slang http://www.urbandictionary.com/define.php?term=oh%20boy "wasn't it great" is a positive phrase. this person thought it was great. 2. this is stating that you are tired, "boy" is not needed, it is more slang i would say.