Question: Who was the main rival of Johnny Torrio and Al Capone during the Chicago bootlegging wars of the 1920s?

Context paragraphs:
Lexington Hotel: The Lexington Hotel was a ten-story hotel in Chicago at 2135 S. Michigan Avenue that was built in 1892 (or 1891) for attendees of the Columbian Exposition.  The hotel is notable for being Al Capone's primary residence from July 1928 until his arrest in 1931.  After the Saint Valentine's Day Massacre, some commenters called the hotel "Capone's Castle."  It was later renamed "The New Michigan Hotel" and functioned as a brothel with 400 rooms.  The hotel closed in 1980.  On April 21, 1986 locked vaults found in the hotel were subject to a live television program called "The Mystery of Al Capone's Vaults", which received 30 million viewers.  The building was demolished in 1995, in spite of the building being landmarked.  The location where the hotel once stood is currently the site of a 296 unit residential high rise called "The Lex" that was completed in 2012.

Dean O'Banion: Charles Dean O'Banion (July 8, 1892 – November 10, 1924) was an American mobster who was the main rival of Johnny Torrio and Al Capone during the brutal Chicago bootlegging wars of the 1920s.  The newspapers of his day made him better known as Dion O'Banion, although he never went by that first name.  He led the North Side Gang until 1924, when he was murdered, reportedly by Frankie Yale, John Scalise and Albert Anselmi.

Due mafiosi contro Al Capone: Due mafiosi contro Al Capone (Spanish: "Dos contra Al Capone" , literally "Two Mafiamen against Al Capone") is a 1966 Italian-Spanish gangster-comedy film directed by Giorgio Simonelli.

Big Jim Colosimo: Vincenzo Colosimo (February 16, 1878 – May 11, 1920), known as James "Big Jim" Colosimo or as "Diamond Jim", was an Italian-American Mafia crime boss who emigrated from Calabria, Italy in 1895, and built a criminal empire in Chicago based on prostitution, gambling, and racketeering.  He gained power through petty crime and by heading a chain of brothels.  From about 1902 until his murder in 1920, he led a gang that became known after his death as the Chicago Outfit.  Johnny Torrio was an enforcer whom Colosimo imported in 1909 from New York and who seized control after his death.  Al Capone, a Torrio henchman, allegedly was directly involved in the murder.

North Side Gang: The North Side Gang, also known as the North Side Mob, was the dominant Irish-American criminal organization (although a large number of Polish-Americans were members as well) within Chicago during the Prohibition era from the early-to-late 1920s and principal rival of the Johnny Torrio–Al Capone organization, later known as the Chicago Outfit.

Black Hand (Chicago): Black Hand extortion was a criminal tactic used by gangsters based in major cities, in the United States.  In Chicago Black Hand extortion began around 1900 and had all but faded away by 1920, and the "Mafia" replaced it.  The Mafia was initially organized by Johnny Torrio, and somewhat organized by Al Capone.

Peter Gusenberg: Peter Gusenberg a.k.a. ""Goosey"" (September 22, 1888 – February 14, 1929) and his brother Frank were German-American contract killers and members of Chicago's North Side Gang, the main rival to the Chicago Outfit.  Peter Gusenberg participated in an infamous attack on Al Capone during a vicious gang war.

Ralph Capone: Ralph "Bottles" Capone, Sr., (January 12, 1894 – November 22, 1974) was a Chicago mobster and an older brother of Al Capone and Frank Capone.  Ralph Capone got the nickname "Bottles" not from involvement in the Capone bootlegging empire, but from his running the legitimate non-alcoholic beverage and bottling operations in Chicago.  Further family lore suggests that the nickname was specifically tied to his lobbying the Illinois Legislature to put into law that milk bottling companies had to stamp the date that the milk was bottled on the bottle.  He was most famous for being named by the Chicago Crime Commission "Public Enemy Number Three" when his brother Al was "Public Enemy Number One".

Paul Kelly (criminal): Paul Kelly (born Paolo Antonio Vaccarelli; December 23, 1876 – April 3, 1936) was an Italian immigrant who founded the Five Points Gang in New York City after starting some brothels with prize money earned in boxing.  It was one of the last dominant street gangs in New York history; Kelly recruited young men who later became prominent criminals of the early 20th century, including Johnny Torrio, Al Capone, Lucky Luciano, Meyer Lansky, Bugsy Siegel and Frankie Yale.  Kelly was said to support election of Democratic Tammany Hall politicians with his gang's activities at elections.

Johnny Torrio: John Torrio (January 20, 1882 – April 16, 1957) also known as "Papa Johnny", "The Fox", and "The Immune", was an Italian-American mobster who helped build a criminal empire, the Chicago Outfit, in the 1920s; it was later inherited by his protégé, Al Capone.  He also put forth the idea of the National Crime Syndicate in the 1930s and later became an unofficial adviser to the Genovese crime family.

Supporting paragraphs: Dean O'Banion Johnny Torrio
Answer: Charles Dean O'Banion