O'Keefe and his teenage son survived the crash with broken bones and other injuries, former NASA spokesman Glenn Malone said. The O'Keefes spent Monday night on the mountain with several volunteers who discovered the wreckage and tended to the injured until rescuers arrived Tuesday morning.
Stevens and O'Keefe are longtime fishing buddies who had been planning a trip near where the float plane crashed.
The crash was a stunning event in a state where Stevens became the most beloved political figure in Alaska history during his 40 years in the Senate, earning a reputation as a tireless advocate for projects that brought millions of federal dollars to the state. He was 86.
"Last night, Alaska lost a hero and I lost a dear friend," Alaska Sen. Lisa Murkowski said in a statement. "His entire life was dedicated to public service from his days as a pilot in World War II to his four decades of service in the United States Senate. He truly was the greatest of the Greatest Generation."
The plane crashed into a brush- and rock-covered mountainside sometime Monday night, authorities said. Volunteer pilots were dispatched to the area around 7 p.m. local time after the plane was found to be overdue at its destination, and they came upon the wreckage about a half hour later.
The weather soon took a turn for the worse, with heavy fog, clouds and rain blanketing the area and making it impossible for rescuers to arrive until after daybreak Tuesday. O'Keefe, his son, and two others were flown to the hospital.
The bodies of Stevens and the other four victims remained at the scene Tuesday, investigators said at a briefing in Juneau. Temperatures ranged from about 48 degrees to 50 degrees overnight at Dillingham.
Alaska National Guard spokesman Maj. Guy Hayes offered no details about the conditions of the survivors or their identities.
National Transportation Safety Board spokesman Ted Lopatkiewicz said the agency is sending a team to the crash site outside Dillingham, located in Bristol Bay about 325 miles southwest of Anchorage. The aircraft is a 1957 DeHavilland DHC-3T registered to Anchorage-based GCI.
The cause of the crash was not immediately known, but the flights at Dillingham are often perilous through the mountains, even in good weather.
Federal Aviation Administration spokesman Mike Fergus said the plane took off at 2 p.m. Monday from a GCI corporate site on Lake Nerka, heading to the Agulowak Lodge on Lake Aleknagik.
The lodge is made of logs and sits on a lake, and photos show a stately main lodge room with a large imposing stone fireplace, a leather sofa and a mounted caribou head on the wall.
Fergus said the plane was flying by visual flight rules, and was not required to file a flight plan.
Stevens was appointed to the Senate in 1968 and served longer than any other Republican in history. He was revered as a relentless advocate for Alaska's economic interests.
Stevens was one of two survivors in a 1978 plane crash at Anchorage International Airport that killed his wife, Ann, and several others. He remarried several years after the crash - he and his second wife, Catherine, have a daughter, Lily.
The airport in Anchorage is now named after Stevens.
"A decorated World War II veteran, Sen. Ted Stevens devoted his career to serving the people of Alaska and fighting for our men and women in uniform," President Barack Obama said in a statement. "Michelle and I extend our condolences to the entire Stevens family and to the families of those who perished alongside Sen. Stevens in this terrible accident."
Over the years, Stevens directed billions of dollars to Alaska.
But one of his projects - infamously known as the "Bridge to Nowhere" - became a symbol of pork-barrel spending in Congress and a target of taxpayer groups who challenged an appropriation for hundreds of millions of dollars for bridge construction in Ketchikan.
Stevens' standing in Alaska was toppled by corruption allegations and a federal trial in 2008. He was convicted of all seven counts - and narrowly lost his Senate seat to Democrat Mark Begich in the election the following week.
But five months after the election, Attorney General Eric Holder sought to dismiss the indictment against Stevens and not proceed with a new trial because of prosecutorial misconduct by federal prosecutors.
Plane crashes in Alaska are somewhat common because of the treacherous weather and mountainous terrain. Many parts of the state are not accessible by roads, forcing people to travel by air to reach their destinations.
Begich's father, Nick Begich, who was Alaska's only congressman in 1972, was killed when his plane disappeared over Alaska with then-House Majority Leader Hale Boggs of Louisiana.
The Stevens crash is the latest in a long line of aviation accidents to claim political figures over the years in the U.S., including Pennsylvania Sen. John Heinz in 1991, South Dakota Gov. George Mickelson in 1993, Missouri Gov. Mel Carnahan in 2000 and Minnesota Sen. Paul Wellstone in 2002.
O'Keefe, 54, was NASA administrator for three tumultuous years. He was deputy director of the Office of Management and Budget when President George W. Bush asked him in late 2001 to head NASA and help bring soaring space station costs under control.
But budget-cutting became secondary when the shuttle Columbia broke apart during re-entry in 2003.
O'Keefe's most controversial action at NASA was when he decided to cancel one last repair mission by astronauts to the Hubble Space Telescope. He said the mission was too risky. His successor overturned the decision. The Hubble mission was carried out last year.
O'Keefe left NASA in 2005 to become chancellor of Louisiana State University. He is now the CEO of defense contractor EADS North America and oversees the bid for the hotly contested Air Force refueling jet contract.
The contract competition, which pits EADS against rival plane maker Boeing Co., is for a piece of what could eventually be $100 billion worth of work replacing the military's fleet of aging tankers.
Associated Press writers Mark Thiessen in Anchorage, Alaska, and Pauline Jelinek, Matt Apuzzo and Natasha Metzler in Washington, D.C., contributed to this report.