what was the purpose of this question?
This is pretty interesting. It really depends on the inflection used with the words. Like if a kid says to his father "Today at school we made volcanoes!", and the dad says "Oh, did you now?", he sounds interested and believes him. But if a one of those jerks at a party says "I totally climbed Mt Everest last week" to which someome sarcastically says "Oh, did you now?" then it means that I really don't believe you. I guess the biggest difference is the length of the pause after Oh.