I chose "oxygen" as a medium difficulty word because it contains an uncommon letter "x" which makes it somewhat challenging, while still having regular vowels like "o" and "e". At 6 letters, it's the perfect length for Hangman - not too short but not overly long. It's a word that most people know, but not one that would be among the first guesses in a game of Hangman. The letter pattern is also interesting with no repeated letters.