The former Bedford School pupil was astonished to receive a D in one of his six history modules. He had won the prize for the best history student in his year for seven years running at the private school in Bedford.
He had achieved As in English and economics but the result brought his overall history mark to a B, which cost him a place last year to read history at Brasenose College.
Oxford held the place open for him while he appealed against the marking, first through the Oxford, Cambridge and RSA board and then the independent Examinations Appeals Board.
In March, the board agreed unanimously to recommend the exam board re-mark the paper, under the supervision of an appeals board member, raising Mr Herring's overall history result from a B to an A. And at the end of May he got his improved mark, 292 days after receiving his first grade.
Mr Herring, now 19, is on a gap year and was this week teaching cricket and hockey at a school in Pretoria, South Africa.
He said: "This is fantastic. I'm delighted, especially after waiting so long."
His father, Stephen, paid tribute to the school,whose fees range from pound;6,870 to pound;16,755 per year, for supporting his son, and the appeals board for its professionalism.
But he said: "I wonder how many other sixth-formers who did OCR A-level history had the wrong grades, and either through pressure of time or through having too much faith in the system have not come forward."
* The grades of an Eton College student on the same paper were also raised from a U to an A following a ruling by the appeals board. This result took almost a year.