in England we take our exams in the may/June and get results in the August of the year we want to go to uni, but for Oxford you need to apply in the September/October before that. so I applied last October and will be going to uni this October provided I get the grades. if I don't meet the requirements I will go to my back up uni or reapply.
so you should apply October 2018 (deadline is Oct 15th 6pm UK time), interviews would be December 2018, find out if you get an offer in January 2019 then hope to meet the grades when you sit your exams in June.
How do you think you'll do?