The operating expenses of a business a routine direct or indirect expense to keep the routine operations up. The are incurred after the project starts, i.e. after commissioning. They are short-term in nature and can be identified with a particular sale or a particular period. Please open file v21 from https://drive.google.com/file/d/0B0F5ZCsTvPP2RGNXQ25qZjcyUTQ/view?usp=sharing have a look at the Opex sheet and then watch the video.