In Oregon, a workers' compensation claim can be reopened for an aggravation within five years of claim closure. For example, if a worker sustained a back strain on January 1, 2010, he may treat for six months, at which point he would be medically stationary. If the claim was then closed on June 1, 2010, he would have "aggravation rights" until June 1, 2015. "Aggravation rights" simply refers to the right to have a claim reopened for an aggravation for five years after claim closure.
However, a claim will not be reopened automatically. An aggravation claim must be filed. This is usually in via an 827 form which is completed at a doctor's office. The form is sent by the doctor to the insurer or the State of Oregon. The insurer then has 60 days to accept or deny the aggravation claim, just like an original injury.
The legal test for whether an aggravation has occurred is whether the accepted condition has "pathologically worsened." This is something the medical expert (i.e., your doctor) has to determine. A good example of an obvious aggravation would be a meniscus tear of the knee needing a new surgery because it re-tore.
One issue that commonly comes up is when the accepted condition is not something that can pathologically worsen. For example, if the claim was originally accepted for a lumbar strain, it can be very difficult to prove a worsening. This is because strains, by their medical definition, will heal within a period of 3 to 6 months. If you still have back pain a year or more after the injury, it is likely that you have a different condition, rather than a strain. So, if your claim was originally accepted for a strain and closed, it can be hard to have an aggravation because the strain should have fully resolved. In that situation, you likely have a new claim, rather than an aggravation of the original claim.