English-dominated companies will be like English newspaper (The Gazette, The Global and Mail), magazines, theaters or probably those international companies.

If you are going to school, on campus job will be the best, since all you need is English.

If you are looking part-time sales, then learn some sentences in French related to that kind of job, you may show that you know some basics in the interview if they ask. Your working experience at Ogilvy should help you to find related job, then try HR as you wanted.

This may also help:
"For more immediate assistance navigating the barren English job market, try Youth Employment Services (YES) Montreal, located at 666 Sherbrooke W. YES Montreal offers a wide range of services, including job search tools, resume-building workshops, self-employment strategies and French classes. With a diverse list of funders including the federal and provincial governments, YES is a non-profit organization, and program fees range from free to definitely affordable." (Mirror)

http://www.yes.on.ca/
http://www.yesmontreal.ca/yes.php

Good Luck and Welcome back to MTL!