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This site says, of the male name Otto: "Later German form of Audo or Odo, originally a short form of various names beginning with the Germanic element aud meaning "wealth, fortune"." It also gives Aud as a Norwegian female name, as you say, and links it to Otto.

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This makes me think that your father's name looked masculine to his parents, so he (and you) got Aud and your aunt got a very feminised version of it: the modern-looking Audra. I hope that helps!