Her name is Ran. People call her 'Oran' probably out of respect or as an endearment (my guess is that someone like Akio started it as a nickname and the rest followed). The o- prefix is used for female names, particularly historically. Like if you've read historical manga, you'll notice they often put an o- in front of all the girls' names. I would've used 'O-Ran' so that people could recognise that it's a prefix, but many of the younger characters also add a -san at the end too, so it would look funny to make it 'O-Ran-san'. =/ You can read a bit about it here: http://en.wikipedia.org/wiki/Honorific_speech_in_Japanese#Female_names