it's horrible when things like this go wrong and I sympathise strongly. I was in a similar position last year, but it was corrected before the deadline of October the 15th.
I would suggest to first see what your remark is. Because Oxford is Oxford - you'll be up against a majority of people who did get an A in history and that's all the tutors can see. That, despite everything, you didn't get an A in a subject that you'd be studying at one of the best universities in the world...whilst they have these people that did. The first thing we got told last year at school was that have a minimum of 3As at as level and do not apply for the subject in which you didn't get an A.
I'm not the best to advise on matters as this - but I have been through the application process. If you meet the entry requirements (3As), you have a chance since Oxford relies heavily on admissions tests and interviews additionally. However you're in the strange position of technically meeting the offer, but not.
I really hope it turns out well for you.