Usage examples for oath:

• And you can't- you are on your oath remember! "The Orange-Yellow Diamond" – J. S. Fletcher

• Nobody in Carlingford had ever before heard an oath issue from the respectable lips of the clerk of St Roque's. "The Perpetual Curate" – Mrs [Margaret] Oliphant

• I take oath now, sir, never to eat again as long as I live. "Green Fancy" – George Barr McCutcheon