Witness presumed to speak truth; jury judges of credibility. A witness is presumed to speak the truth. This presumption, however, may be overcome by the manner in which the witness testifies, by the character of the testimony of the witness, or by evidence affecting the character or motives of the witness, or by contradictory evidence. Where the trial is by the jury, they are the exclusive judges of the credibility of the witness

ORS 44.370 — under Chapter 44.