Most child support awards in Oregon are based on a guideline calculation, which in turn is based primarily on the gross incomes of both parents. You can access the calculator here: http://www.dcs.state.or.us/calculator/
In order to have your support reduced because of your job loss, the current judgment must be modified. You can hire an attorney to commence modification proceedings, or you can seek administrative modification through the Division of Child Support. Both processes can take time, so if you foresee a long period without work, you should act promptly. You can access the necessary DCS forms here:
http://www.dcs.state.or.us/forms/default.htm