He was born in Chisholm, and still owns his original family home in Chisholm. He received his B.A. at the College of St. Thomas (now the University of St. Thomas in St. Paul. He also received a Master's degree in European Studies from the College of Europe in Bruges, Belgium, with further study at Laval University in Sainte-Foy, Quebec, Canada and Georgetown University. He spent four years as a civilian language teacher in the United States Marine Corps, teaching English to Haitian military personnel and French to American Marine officers and noncommissioned officers.

Oberstar served as chief staff assistant to 8th District Congressman John Blatnik for 12 years. When Blatnik didn't run for reelection in 1974, he endorsed Oberstar as his successor. Oberstar won handily in November and has been reelected 15 times without serious difficulty. The Democrats--in the persons of Blatnik and Oberstar--have held the seat since 1947. His lowest winning percentage was 59 percent in 1992, but since then has not earned less than 65 percent of the vote. He was elected to his 16th term in 2004, making him the longest serving member of either house of Congress in Minnesota's history.

Oberstar is an internationally recognized expert on aviation and aviation safety. He has served on the Transportation and Infrastructure Committee since his first term, and is currently its ranking Democrat. Blatnik had chaired the committee during his last two terms in Congress (when it was known as the Public Works Committee), with Oberstar as staff administrator. Oberstar was a member of the President's Commission on Aviation Security and Terrorism (PCAST) which was set up in September 1989 to review and report on aviation security policy in the light of the sabotage of Pan Am Flight 103 on December 21, 1988. An avid cyclist, Oberstar has been a champion of creating trails for cycling and hiking to promote healthy living and more active lifestyles. In 2005 he authored and had passed the Safe Routes to Schools act, a $200 million program that helps school districts address the growing problem of childhood obesity by building biking and walking paths to schools, hiring crossing guards and promoting safety programs.

Oberstar generally has a liberal voting record, but is pro-life on abortion and opposes gun control. The latter stand is fairly common among DFLers, as Minnesota is a "pioneer state" with a long hunting and fishing tradition and many DFL officials are hunters and anglers.

Oberstar resides with his wife, Jean, in both Potomac, Maryland, a suburb of Washington, D.C., and in his original family home in Chisholm, Minnesota.