Do I have a right to overtime pay even if my job expects me to sign a paper saying otherwise?

Hey there, I work fastfood in Ontario, and we're getting three paychecks for the month of June and I want to get myself more savings to put away by working overtime. I know a lot of places don't like to give overtime and I heard that some places even get their employees to sign a paper saying that they give up their time and a half for ot. Even if they do sign, would they still be entitled to it if it was illegal for the employer to waive it?