oh my gosh is a minced oath version of oh my god. The latter would be Oh, mein Gott! in German, the former can't be translated, but you could use a rough equivalent.
Ach, du meine Güte.
Ach, du heiliger Bimbam.
oh nein
The German translation of Oh snap! is:Mist!Verdammt!
really - wirklich
English: "oh" is German "o". Chrismas song: "O du fröhliche, o du selige ..."
Oh je, deine Hosen brennen.
I am really in love with you is Ich bin wirklich in dich verliebt in German.
ANSWER Ach, mein Gott!
Ja, wirklich.
If you mean really as in very, i.e. He is really stupid, the you say tres. If you mean really as in "Is that true?" or "Oh, really?", then you say c'est vrai?
Really as in: Wow, really? is "wirklich" Really as in: That's really cool. is "echt".
Irish - i ndáiríre?
If you really want to translate it is " RnB für immer" but most German people will just say "RnB forever".