Ou-dis-sun is a mispronunciation of the Hudson River.  It is a clue to the actual setting of the story, which is New York City after a terrible bombing.  The short story takes place many, many years after the destruction, and the names of places have changed over time.  It is possible that survivors after the blast were left with a speech impediment as part of their injuries, and so words became slurred or changed.  Written language became the exception rather than the norm, and so word of mouth can change the pronunciation of words.  For example, when French trappers settled in the western New York area, they came across the Niagara River and remarked, "Beau fleuve!", which means "beautiful river".  The settlement they established was also called Beau Fleuve, but over time, the French words were slurred together to create what is now known as Buffalo, New York.  The city of Buffalo is not named for a large plains animal that never even lived there.  That is how Hudson could have become Ou-dis-sun.