Common law—also known as case law, court opinions, judicial precedent, or judge-made law—is the body of law within each state or federal circuit that is created when judges write decisions (court opinions) resolving lawsuits—and in doing so sometimes apply legal principles that are not included in the state or federal statutes. When the state legislature or the U.S. Congress has not passed a law (state or federal statute) governing a certain legal issue, the courts look to traditional (and sometimes new) legal principles to resolve those issues. And those principles—as stated in case law—are the common law.

The concept of common law is derived from the English system and is based on precedent—the manner in which courts have previously resolved a similar legal dispute involving similar factual circumstances. If a similar legal dispute involving similar facts has been resolved in a certain way in the past, a current court is generally required to follow the prior application of the law under the doctrine of stare decisis—a Latin term pronounced “starry duh-sigh-suss”—to maintain consistency and predictability in the law. But a court’s obligation to follow precedent is generally limited to case law from higher courts (generally the state supreme court or the U.S. Supreme Court) or from the same court (district court of appeals or federal circuit court, for example).

And if a court determines a dispute involves a legal issue that is not addressed in a statute or in a previous court opinion from the same state or jurisdiction (and thus is a matter of first impression) the court will resolve the dispute as best it can—often by analogy to similar statutes and legal principles—as well as the more subjective ideas of fairness and justice.

In Oregon, common law refers to the body of law developed through judicial decisions rather than through legislative statutes. When Oregon's state legislature has not enacted a statute that covers a specific legal issue, courts rely on common law principles to resolve disputes. These principles are derived from previous court decisions, particularly those from higher courts such as the Oregon Supreme Court or the U.S. Supreme Court, under the doctrine of stare decisis. This doctrine mandates that courts follow established precedent to ensure consistency and predictability in the law. However, when an Oregon court encounters a legal question that has not been previously addressed (a matter of first impression), it must use its best judgment to resolve the issue, often drawing on analogous statutes, legal principles, and considerations of fairness and justice.