1) abbreviated form of "Oh My God", said when people pray.
2) abbreviation of the name of an anime/manga Oh! My Goddess, sometimes also known as Ah! My Goddess
3) abbreviation of "Oh my god", said by over-excited, shocked, horrified people.
1) OMG, I pray that my goldfish does not die.
2) Hey could I please borrow your OMG manga?
3) OMG!!! I just saw a flying pig!!!