A single-engine Piper Malibu crashed during approach at a regional airport which serves the Experimental Aircraft Association's AirVenture airshow, said event spokesman Dick Knapinski.
Each year the weeklong event draws about half a million people from around the world and approximately 10,000 aircraft.
Many attendees were arriving in anticipation of an afternoon flight demonstration by the Pentagon's new ultra-advanced F-35 Lightning II fighter jet.
No one was hurt on the ground in the crash, Knapinski said, and federal officials were investigating, as they do in all aviation incidents.
Those on board were identified as: Kenneth J. Kaminski, 46; Gerald T. Kaminski, 71; Margaret C. Laing, 30; Nathan P. Gargano, 26, and Neil F. Dill, 56.
Four of the occupants were able to leave the plane on their own after the accident, while one -- it wasn't clear who -- was pulled out by rescue workers. The conditions of those injured was also not immediately clear.
The accident forced closure of Wittman Regional Airport for about two-and-a-half hours, Knapinski said
At around 8 a.m., a rising dark cloud of smoke signaled to people attending the aviation event that something was wrong, said George Widener, who witnessed the smoke.
"Everybody pretty much wanted to know what happened," Widener said. "Smoke like that is never good at an airshow. I was hoping nobody got hurt, hoping everyone was OK."
When the airport re-opened, a long line of small aircraft lined up on an airport taxiway, waiting to depart.
Accidents like this aren't unheard of at big airshows, said Widener, 44, who said he's attended the Oshkosh airshow every year since 2004.
Every year the Federal Aviation Administration dispatches special teams of air traffic controllers to Oshkosh for the airshow to ensure the safe handling of up to 1,000 separate operations daily, according to the Federal Aviation Administration.
Both the FAA and officials from the National Transportation Safety Board -- who were already on site for the event -- were investigating the crash. The plane's FAA identification number -- provided by Knapinski -- said the plane was manufactured in 1984 and was registered to an owner in Stevensville, Michigan. The plane's certificate was valid and was set to expire in 2016, according to FAA records.
General aviation pilots and enthusiasts from around the world make their way every year to Oshkosh to see the latest experimental aircraft and advanced aviation technology.
The airport closes each day to commercial and general aviation traffic during airshow performances, and the FAA establishes a restricted airspace around the area.