The three remaining shuttles have been grounded since Columbia broke up on re-entry, killing all seven astronauts, on 1 February last year.
The return to shuttle flight is set for March 2005, but O'Keefe has previously said safety recommendations issued by those who investigated the shuttle disaster will be followed to the letter, regardless of schedule constraints.
The cost of implementing the recommendations of the Columbia Accident Investigation Board would be spread over three years, O'Keefe said.
NASA spent $94 million on upgrades in fiscal 2003. Plans call for spending $265 million in fiscal 2004 and $235 million in 2005.
"In total, we're looking at a projection that's going to be on the order of six to seven hundred million dollars, roughly, to implement all those findings and recommendations," O'Keefe told members of the Senate commerce, science and transportation subcommittee.
The investigators recommended extensive changes including improvements to the shuttles' thermal protection systems and the addition of a way to inspect and repair damage to the outside of the shuttles while they are in orbit.
They also told NASA it must fix its "broken safety culture." To do that, they urged NASA to set up an independent technical engineering authority separate from the rest of the shuttle team to monitor safety, reporting directly to space agency headquarters.
O'Keefe said his cost estimate included the establishment of this technical authority.
It also included the cost of replacing parts on all three shuttles to fix a recently discovered 20-year-old error. Gears were installed backward on the speed brakes in shuttle Discovery's tail section.