Generally, the injured worker has to prove a workers' compensation claim in Oregon. However, this depends on the issue at hand.
When you file a claim for workers' compensation benefits, the insurance company investigates the claim, and decides whether there is sufficient proof that you suffered an injury as a result of an on-the-job activity. An employer or an insurance company need only have a "legitimate doubt" about the claim in order to deny it outright. Some people, including myself, think that many insurance carriers will deny a claim knowing full well that it should be accepted, hoping that the injured worker simply walks away, and does not appeal the denial.
After a claim is denied, the injured worker can file a request for hearing. At the hearing, the injured worker must prove that he or she suffered an injury that occurred in the course of employment and arose out of a work activity. Lawyers refer to this as the "burden of proof." In many cases, the injured worker does not have to prove a specific medical problem resulted from the work injury, but that the work injury event either caused disability from work, or the need for medical care.
In some cases, the evidence may show that the injured worker had a pre-existing condition, and that the injury event combined with the pre-existing condition. This is called a "combined condition." The insurance company must cover these kinds of claims as long as the injury event (the fall, strain, or other injury event itself) is the major cause of the combined condition.
In many cases, the burden of proof is now on the employer to show that the injury component of the combined condition never was the major cause of the combined condition, or is no longer the major cause of the combined condition.
Obviously, who has the burden of proof is key because it may determine whether you get benefits at all.