What grades are needed for a Canadian high school student to have a reasonable chance of admission to Oxford?

Or Cambridge. I realize that it is more than just grades, like personal statement, letter of reference, community service, interview, UKCAT score, etc., but what grades are typically needed? I don't understand the UK system of high school courses and grading, so it is hard to translate UK academic achievements into Canadian equivalents. The Oxford admissions page only says Canadians need >85% to be considered, but we all know it would likely have to be much higher than that.