The translation of "go ahead" depends on the context it is used, in the sense of "to continue" it is "fortfahren" or "weitergehen", as instruction "Go ahead!", it can be translated as "Komm schon!" or "Mach schon!" For "Oh my god" you can say: "Ach du lieber Gott" "Oh mein Gott" "Ach du liebe Zeit" "Du liebe Zeit" "Ach du liebes bisschen" "Du liebes bisschen" "Um Gottes willen"

go ahead has really many meanings. Have a look to this site: http://www.dict.cc/?s=go+ahead Another one for "Oh my god" is "Ach Du liebe Güte".

It depends on context.