The airplane involved was United Express CRJ 700, operated by Mesa Airlines, which was on a flight from Dulles Airport in Washington to Boston's Logan International.
Authorities say the plane had a problem with the onboard oxygen generator shortly after takeoff.
The plane landed at the New Castle Airport without incident.
The 61 passengers and the four crew members were not injured.
Buses supplied by the Delaware Air National Guard took everyone to the terminal.
Shashi Bellamkonda gave Action News permission to use a photo he sent via Twitter of himself and a fellow passenger with their oxygen masks on after the landing.
Twitter user Kevin Fawley tweeted a similar photo, along with pictures of the scene.
A new plane took most of the fliers to their destination.