Trivia: The ending was originally different. After the scene where Derek is crying over Danny, it cut to Henry being arrested, then his family in the kitchen mourning him and saying he was a good boy, and then Derek shaving his head again while smiling. When Edward Norton read the script, he said "I thought the movie was supposed to be anti-Nazi." So the ending was soon changed.
Trivia: Edward Norton drank purified tuna to bulk up for his role of Derek Vinyard.
Answer: The tattooed DOC on gang members' arms means Disciples of Christ - the gang that they belong to in the movie.