It's true! Thanks to the University of Ottawa's Regulation on Bilingualism, you may write your assignments and exams in French or English in nearly any course. While we encourage Immersion and Extended French Stream students to practice writing in French as much as possible, we recognize that, in some circumstances, writing in your second language can make a big difference, so the option is always open for you to write in English as a safety net.
For example, many French Immersion and Extended French students choose to write shorter assignments in French, and longer or more difficult assignments in English. This gives them the opportunity to practice writing in French without as much risk. You should also make use of the Academic Writing Help Centre when preparing your written assignments in either language.
Note that, in language courses (FLS courses, for example), you have to write in the language being studied. For example, you cannot write your FLS assignments in English, just like you cannot write in English if you are learning German!