I’m going to have to apply to unis soon (not this year) but I was wondering: if I want to double major in Computer science (or Marketing) and English, what would I choose on the OUAC application? Am I supposed to choose the math/comp (or business) department or like arts/humanities?Will I be able to do this in 4 years?Thank you 
In Ontario, the business majors are set up as a separate programs. In Ontario, they are called commerce. You can’t double major outside of them. In addition, at many of the universities, U of T, for example, you can apply to either the commerce program or a different program, but not both. So, if you decide you want to do commerce at U of T (called Rotman Commerce), you can’t apply to computer science or English at the St. George campus.
You could theoretically double major in computer science and English. Although you would apply to and be admitted under a program (humanities or comp sci in this case) in your first year, you wouldn’t register as a major until the end of your first year. So, theoretically, you could apply as a computer science major, register for all English classes and then register as an English major at the end of your first year. There are a couple of slight issues for this - some majors (such as comp sci) might have grade requirements to get into the major at the end of your year. The grade requirements are usually in the B to A minus range. Also, when it comes time to register for classes in the summer, people admitted under certain programs get first shot at registering for the courses in that program. So, if you apply as an English major and then try to register for comp sci courses, you may not be able to register for a week or two after the people admitted under comp sci registered.
Clear as mud, right? I recommend googling the admission requirements for comp sci, commerce, and English programs at the universities you are interested in. Each university is somewhat different. I used U of T as an example, but I know that other Ontario universities do it differently. Waterloo, for example, allows you to apply to several programs at the same time. If you are interested in commerce or computer science, you will need calculus by the end of grade 12. These two programs are probably the most competitive (next to engineering) in Ontario universities. For commerce, your grades will need to be low 90’s or higher. For computer science, mid 80’s should probably do it. Again, these grade requirements may be higher or lower depending on the university. While you’re googling, check out the admission averages for each program at the different universities. There is a wide range.
Good luck!
Thank you so much!! Let me get this straight: if I were to, say, choose English/humanities on the university application, I can still take, say, computer science courses and potentially earn a double major/minor and vice-versa?
Yes. Keep in mind that even if you apply to a humanities program and then want to take computer science courses or a major, you will need the prerequisites as if you had a applied under the computer science program. Specifically, you’ll need calculus if you want to take anything about the most introductory comp sci course.