Okla. Stat. tit. 22, § 22-745: Murder, burden of proof in mitigation of

Upon a trial for murder, the commission of the homicide by the

defendant being proven, the burden of proving circumstances of

mitigation, or that justify or excuse it, devolves upon him, unless

the proof on the part of the prosecution tends to show that the

crime committed only amounts to manslaughter, or that the defendant

was justifiable or excusable.