Oral allergy syndrome (OAS) or pollen-food allergy syndrome (PFAS) is a type of allergy classified by a cluster of allergic reactions in the mouth and throat in response to eating certain (usually fresh) fruits, nuts, and vegetables. It typically develops in adults with hay fever. It is usually not serious.

OAS is the result of cross-reactivity between antigens of tree or weed pollen and antigens found in certain fruits and vegetables. Therefore, OAS is only seen in people with seasonal pollen allergies, and mostly people who are allergic to tree pollen. It is usually limited to ingestion of uncooked fruits or vegetables. In adults, up to 60% of all food allergic reactions are due to cross-reactions between foods and inhalative allergens.

OAS is a class II allergy where the body's immune system produces IgE antibodies against pollen; in OAS, these antibodies also bind to (or cross-react with) other structurally similar proteins found in botanically related plants. This differs from class 1 food allergy where sensitisation to the protein occurs in the GI tract, and is produced as a result of exposure to the food itself.

OAS can occur any time of the year, but is most prevalent during the pollen season. Individuals with OAS usually develop symptoms within minutes of eating the food.