Two prisoners, Lester Burns and Michael Coleman, were at large following the Tuesday afternoon escape at Weatherford. Six were rounded up shortly after police put out a call asking for help. A shotgun aboard the van was also recovered.
The inmates were from jails in the western and northern United States and were being transported among agencies in the same regions of the country.
One or two fell ill near Weatherford, which is about an hour west of Oklahoma City on Interstate 40, Weatherford Police Chief Byron Cox said. His assistant, Louis Flowers, said the inmates had been left in a locked compartment inside the van.
"Two of the prisoners kicked the partition out and were able to gain access to the front of the van. ... The keys were still in the van as far as we know, and it was still running," Flowers said.
Cox said the inmates stopped a mile down the road and two ran away. Dogs and helicopters were summoned to aid in the search.
"We're going door-to-door and searching every residence that we can," Flowers said.
Oklahoma Highway Patrol spokeswoman Betsy Randolph said officers recovered a 12-gauge shotgun that had been inside the vehicle. No injuries were reported.
Authorities said Coleman was last seen wearing a black T-shirt with a teddy bear on it and black shorts. Burns was wearing a camouflage shirt and black shorts.
No details were available about what the inmates may have been accused or convicted of. Asked what he knows about the inmates, Cox said: "Not much right now, we're still trying to get that right now from the transport company."
Prisoner Transportation Services of Nashville, Tenn., acknowledged it was moving the prisoners but said it could not comment further because the investigation is ongoing.
Critics of private prison transportation companies have complained that they are poorly regulated.
Southwestern Oklahoma State University in Weatherford was locked down for about two hours while police searched for the two inmates. The school lifted the lock down just before 4 p.m., KOKH television reported.