I looked it up on www.truthorfiction.com and it says it's true.
Taken from the site:
The Truth
Lt. Commander Edward Henry "Butch" O'Hare is the subject of many articles that document his outstanding service as a pilot during World War II. He was presented with the Congressional Medal of Honor for his actions against the Japanese and defending the U.S.S. Lexington. According to the official citation of his Medal of Honor, he won the recognition "For conspicuous gallantry and intrepidity in aerial combat..." It says he was the section leader of Fighting Squadron 3 on February 20, 1942. According to an article on aviation-history.com, six Wildcats were sent into the air to protect the Lexington from Japanese bombers. O'Hare and his wingman spotted the enemy planes first. The wingman's guns jammed, however, and the other four planes were too far away, so O'Hare faced 9 twin-engine Japanese bombers alone. He shot down five of them and damaged a sixth before other U.S. fighters arrived. No enemy bombs made it to the Lexington. The Medal of Honor citation calls it "...one of the most daring, if not the most daring, single action in the history of combat aviation..." O'Hare was killed in November of 1943 during the battle for the Gilbert Islands in the South Pacific. He was accidentally shot down by another American plane during a night mission. It is true that Chicago's O'Hare airport is named after him and there is a restored airplane on display there similar to the one that O'Hare flew.
Butch's father, Eddie O'Hare, was an attorney and business partner of the famous gangster Al Capone. He helped run Capone's horse and dog track operation in Chicago. He was described as being devoted to his son. There was a point when Eddie decided to secretly become an informant for the Internal Revenue Service and it was with his help that the government convicted and imprisoned Capone for income tax evasion. Some have said that Eddie became an informant because of a change of heart and a desire to go straight. Others have said it was merely his way of saving his neck in the face of potential prosecution. It was an article in Collier's magazine in 1947 about Eddie O'Hare's work as an informant that helped win public favor for him and the eventual naming of Chicago's airport after his war-hero sun. The article was written by Frank J. Wilson, the Treasury Department investigator with whom O'Hare had worked on the case. The article was titled "Undercover man: he trapped Capone." Wilson called O'Hare one of his best undercover men.