Hi there,
Generally, most of the people who live in Montreal are bilingual. If you are looking for job that is service-based, then French is definitely an asset, since some Quebeckers prefer and feel more comfortable to speak French; so that’s also what most employers are looking for. But, all you need to know is only some basic conversation in French that is related to your job (even a few sentences can help you get the job), and it shouldn’t be too hard to pick up; like me, I only have some basic knowledge of French. But of course, it depends on what kind of job you are looking for. You can try to find job in English-dominated area (especially in West Island and in the Western half of Montreal), or English-dominated companies and schools, or jobs that do not require a lot of communication. If you have a specific experience or skill related to job that you are looking for, then working in Montreal without knowing French is possible. I know immigrants who don’t know any French, but have specific skill or background, work in bilingual companies here. Since you have already worked in Ogilvy, why not try there first, showing that working experience should help you to get job there again.

Good Luck! :)