If the oral allergy syndrome is due to x-reactions between homologous proteins in ingested food and aero-allergens why aren't food skin tests +?

Too complex...?! The gut is both an immune privileged site and responds adversely (and often unpredictably) to ingested materials (e.g. Bacteria, food, dirt, etc.). There may be too many variables to easily answer your question. A case in point is eosinophilic esophagitis. Sadly, given our knowledge gap, too many patients take too many supplements or exclude too many products for questionable reasons.
Need fresh food. Often times the food skin prick test is positive, however fresh food may be needed.
Unstable. The allergens that cause oral allergy syndrome are unstable and will degrade easily on heating and over time in the food extracts.