Evidence is information that exists in physical or digital form—such as written agreements and e-mail communications—as well as testimony from witnesses who have personal knowledge of facts relevant to a dispute. Evidence generally must be relevant to be admitted in court and considered by the factfinder (judge or jury) in resolving a lawsuit. And some forms of evidence, such as expert witness testimony, must be shown to be sufficiently reliable (as determined by the judge) to be admitted in evidence.

In Oklahoma, evidence is crucial in both civil and criminal proceedings and must adhere to the rules of evidence as outlined by the Oklahoma Evidence Code and relevant federal laws if the case is in federal court. Evidence can be in the form of physical objects, digital data, written documents, or witness testimony. For evidence to be admissible, it must be relevant to the case, meaning it should make a fact more or less probable than it would be without the evidence and must pertain to the dispute's resolution. The reliability of certain types of evidence, such as expert witness testimony, is also a key consideration. Expert testimony must be evaluated by the judge for relevance and reliability before it can be admitted. This involves ensuring that the testimony is based on sufficient facts or data, is the product of reliable principles and methods, and that the expert has applied the principles and methods reliably to the facts of the case. The Oklahoma Evidence Code is largely modeled after the Federal Rules of Evidence, and Oklahoma courts often look to federal case law for guidance on evidentiary issues.