Oh, well maybe things have changed then: when both my brother and myself applied it was only final marks. We were able to log in to OUAC and see exactly what marks were being assessed, and none of them were midterms. In first semester it was your top six 11U (unless you had a 12U and then they were included too, regardless of grade) courses, then at the end of first semester when final marks came in it was your top 6 of 11U/12U courses.

My brother is in second year now, so it's not like this was ages ago but maybe they've started accepting midterm marks.

If your midterm grades do get counted, then as long as you're in the 80's range you'll be fine.