The "O" doesn't stand for anything- it's a term of address, like "hey!" However, this exclamation is currently obsolete. Newer translations would probably omit it because its meaning is unknown to most of us.
/oʊ/Pronunciation Key - Show Spelled Pronunciation[oh]Pronunciation Key - Show IPA Pronunciationinterjection, noun, plural oh's, ohs, verb –interjection 1.(used as an expression of surprise, pain, disapprobation, etc.) 2.(used in direct address to attract the attention of the person spoken to): Oh, John, will you take these books?–noun 3.the exclamation “oh.” –verb (used without object) 4.to utter or exclaim “oh.” [Origin: later sp. of O, from mid-16th century