More details are required about your system (terminal units, economiser, DCV etc.) and what exactly you need to hardsize but I'll try to answer to some extent and in general about OA controls. First thing you should look at is the OS:DesignSpecification:OutdoorAir object, which can be found at spaces tab-->properties-->airflow and defines the outdoor air requirements for other objects. More information can be found here. Then, in case you have a VAV ATU (air terminal unit), you should also set the Control For Outdoor Air setting to Yes so as to ensure the minimum outdoor airflow. Keep in mind that there are more options regarding your OA in the ATU settings (e.g. Zone Minimum Airflow Input Method, Constant Minimum Air Flow Fraction, etc.). Then there are some additional control considerations which can be accessed through the OS:Controller:OutdoorAir object found in your airloop in the HVAC tab. The first is the schedule when OA is suplied which is the field Minimum Outdoor Air Schedule Name. Moreover, in this object you can define your economiser operation which can affect your OA supply as well. Lastly, from the control subtab you can switch on demand control ventilation (DCV) which in return supllies OA according to the occupancy schedule. I hope this helps.