The seven anti-government militants, including brothers Ammon and Ryan Bundy, faced charges of conspiring to impede federal employees at the Malheur National Wildlife Refuge through intimidation or force.
The six men and one woman each faced up to six years in prison on the conspiracy charge.
Several also faced gun charges in the high-profile five-week trial.
Defense attorney Lisa Ludwig called verdict "stunning" as the group's supporters erupted in cheers, local news media reported.
But a scuffle broke out in the courtroom after Ammon Bundy's lawyer Marcus Mumford challenged the federal judge's decision not to let his client walk free because of another case pending against him in Nevada.
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US Marshals tackled Mumford as he argued with the judge, taking him into custody while all those in the courtroom were immediately ordered out.
Social media, meanwhile, was buzzing about the verdict with one tweet describing it as "unreal" and another saying it marked "a sad day for America."
"No one could have scripted a more bizarre tale," a Twitter user identifying herself as Anne Kiser said.
Oregon Governor Kate Brown said that while she respected the jury's decision, she was disappointed.
"The occupation of the Malheur Refuge by outsiders did not reflect the Oregon way of respectfully working together to resolve differences," she tweeted.
'Embolden extremist groups'
The Center for Western Priorities, a conservation group, also expressed disappointment with the verdict.
"The outcome of (the) trial will undoubtedly embolden extremist groups," it said in a statement carried by local media. "It's imperative that local, state and federal law enforcement ensure the safety of our land managers."
However, many others hailed the verdict as a major win for ranchers throughout the West.
"It's a tremendous victory for rural America -- a disastrous, humiliating defeat for the corrupt federal government," one of the defendants, Neil Wampler, told The Oregonian newspaper.
The 41-day siege that began at the remote reserve on January 2 put the spotlight on a long-running dispute over millions of acres of western public land and Thursday's verdict was seen as a significant blow to federal prosecutors.
The takeover was led by the Bundy brothers, whose father Cliven Bundy had been involved in a similar confrontation with federal officials in 2014 over cattle grazing on public land in Nevada.
The Oregon takeover ended with the dramatic surrender of four holdouts, including one who threatened to commit suicide in a phone call with mediators that was streamed live.
Police had earlier fatally shot the group's spokesman Robert "LaVoy" Finicum during a traffic stop as he, the Bundys and several others were headed to a community meeting to plead their case.
The question of land rights has been a thorny issue for decades in western US states, where the federal government owns most of the land.
Many conservative politicians and ranchers like the Bundys argue that the land has been mismanaged and should be handed over to states or turned into private property.
