Also I forgot to mention, the best argument that oral pardons are effective is that Presidents have done it before. Gerald Ford, without going through the formal DOJ process, gave Richard Nixon an oral pardon on national television. The exact text of the speech contains this clear and conclusive language:
“Now, therefore, I, Gerald R. Ford, President of the United States, pursuant to the pardon power conferred upon me by Article II, Section 2, of the Constitution, have granted and by these presents do grant a full, free, and absolute pardon unto Richard Nixon for all offenses against the United States which he, Richard Nixon, has committed or may have committed or taken part in during the period from July 20, 1969, through August 9, 1974.”
He had already granted it when Nixon agreed to accept it by telephone. That’s why it says “have granted”
His error on the start date was later corrected in writing but even that document tells us that pardoning can be oral and memorialized in writing. After reciting the Presidents considerations for the pardon, it repeats the paragraph above, corrects the date error, and adds this telling paragraph:
“In witness whereof, I have hereunto set my hand this eighth day of September, in the year of our Lord nineteen hundred and seventy-four, and of the Independence of the United States of America the one hundred and ninety-ninth.”
If the document itself was required to make the pardon effective, then it would not be “in witness whereof”.
This is no different than an oral pronouncement on the record from the bench of a judges order. The absence of a subsequent written order does not make the decision any less effective.