Corey Wyatt, 29, pleaded guilty on Wednesday to charges of being an accessory after the fact and unlawful disposition of a firearm.
Police said he transferred a firearm to David "Joey" Pedersen and helped Pedersen and his girlfriend Holly Ann Grigsby with a stolen vehicle.
Wyatt could face up to 15 years in prison when he is sentenced in April, The Oregonian reported.
Pedersen and Grigsby are awaiting trial on federal charges of kidnapping, carjacking and murder. They are accused of killing a man on the Oregon coast and another man in Northern California.
Pedersen previously pleaded guilty to two counts of aggravated murder for the 2011 slaying of his father and stepmother in Everett, Wash.
Wyatt's 45-minute court appearance was his first in the case. Almost a dozen of his friends and family members, including his wife Kimberly, attended, with some raising their hands when Wyatt entered the courtroom dressed in a black, mixed martial arts T-shirt and basketball shorts.
Assistant U.S. Attorney Jane Shoemaker said Wyatt and Pedersen knew each other in prison. In July 2011, after they were both released and went camping together on the Oregon coast, Pedersen told Wyatt he needed a gun to commit robberies and carry out a white supremacist revolution, Shoemaker said.
She said federal prosecutors were prepared to prove Wyatt's wife, who is not a convicted felon, purchased a 9 mm Lugar pistol. Shoemaker said Corey Wyatt gave it to Pedersen, who used it in an attempted carjacking in Portland in September.
After that, Shoemaker said, Wyatt and Pedersen participated in a mixed martial arts fight in Nevada. After their return, Wyatt took Pedersen and Grigsby to a bus station for a trip to Washington, Shoemaker said.
The couple then stole the Jeep of Pedersen's father and returned to Oregon with his body inside, prosecutors said. They arrived on Wyatt's doorstep in Springfield on Sept. 27, according to Shoemaker.
She said Wyatt got his wife - whose trial on a five-count indictment is scheduled for February - to help dispose of the Jeep by driving to a secluded wooded area. Shoemaker said Pedersen, Wyatt and Grigsby pushed the vehicle over an embankment.
Pedersen and Grigsby stayed with Wyatt that night, then Wyatt took them shopping for camping gear and drove them to the Oregon coast, prosecutors say.
A federal indictment against Pedersen and Grigsby accuses them of kidnapping and killing 19-year-old Cody Myers on the Oregon coast and then traveling to Eureka, Calif., where they killed Reginald Alan Clark.
Kimberly Wyatt is facing charges of conspiracy to unlawfully transfer a firearm to a convicted felon, being an accessory after the fact and other counts.