Oral Allergy Syndrome (OAS): This is the most likely diagnosis given the symptoms of redness, itching, facial edema, and pressure occurring 1 hour after eating a peach. OAS is an allergic reaction to certain fruits and vegetables, including peaches, and is often seen in individuals with pre-existing allergies, such as birch pollen allergy.

IgE-mediated Food Allergy: A classic allergic reaction mediated by IgE antibodies, which could cause symptoms ranging from mild (such as hives) to severe (anaphylaxis). The timing and symptoms are consistent with this diagnosis.
Contact Dermatitis: Although less common, contact dermatitis could cause localized redness and itching if the individual had direct skin contact with the peach. However, facial edema might be less typical unless the peach was in direct contact with the face.

Anaphylaxis: Although the symptoms described do not fully meet the criteria for anaphylaxis (which typically includes symptoms of two or more systems, such as cutaneous, respiratory, cardiovascular, or gastrointestinal), any allergic reaction has the potential to progress to anaphylaxis. It is crucial to monitor the patient closely for signs of worsening, such as difficulty breathing, rapid heartbeat, or a drop in blood pressure.
Angioedema: This condition can cause significant swelling, particularly of the face, and can be life-threatening if it involves the airway. It may occur alone or in conjunction with anaphylaxis.

Exercise-induced Anaphylaxis: This is a rare condition where physical exertion triggers an allergic reaction. Although eating a peach is not typically associated with physical exertion, if the individual engaged in exercise shortly after eating, this could be considered.
Mastocytosis: A condition characterized by an accumulation of mast cells in one or more organs. It can cause a variety of symptoms, including allergic-type reactions. However, this would be an unusual cause of an acute reaction to eating a peach.