Oral allergy syndrome (OAS) or pollen-food allergy is a type of food allergy classified by a cluster of allergic reactions in the mouth and throat in response to eating certain (usually fresh) fruits, nuts, and vegetables that typically develops in adults with hay fever.

OAS is not a separate food allergy, but rather represents cross-reactivity between distant remnants of tree or weed pollen still 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 Type 1 or immunoglobulin E-mediated hypersensitivity, which is sometimes called a "true allergy". 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.

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.