Oregon counties generally follow the state rules as explained in this Mobile Food Unit Guide

What is required for potlucks, barbeques or mobile food sales?

Any establishment operating in connection with any event where food is prepared or served for consumption by the public would require a license, regardless of whether the food is offered for sale or provided free of charge. A temporary restaurant license is required for functions such as fund raisers events advertised to the public, fairs, carnivals, circuses, festivals and concerts. In addition, homemade food offered at such events to the public must be prepared in a kitchen that has been inspected by the local environmental health department. You may review the licensing information pertaining to restaurants and mobile units on the Oregon Health Authority, Public Health website or contact your local environmental health department to review the regulations. Outdoor food sales, such as barbeques and mobile food units that are not operating on property owned/leased by an Oregon Department of Agriculture licensed firm under an employee of the licensed firm, would be subject to regulations and licensing under the Department of Human Services Oregon Administrative Rule 333-162 .Private parties or functions not open to the public do not require a license or permit.