The aim of this study was to assess the prevalence of oral allergy syndrome (OAS) in patients with seasonal allergic rhinitis (SAR) and the possible risk factors for its development. This cross-sectional study was conducted in primary care offices in the Split-Dalmatia County during the period from March 1 to September 30, 2012. Data sources were medical history with confirmation of SAR (positive skin-prick test to seasonal inhalant allergens: grass, tree and weed pollens), anthropometric patient data (age, sex, weight and height), and a questionnaire in which patients evaluated their nasal and ocular symptoms, comorbidities and lifestyle. The χ2-test, Pearson χ2-test, Spearman's rho correlation coefficient and Kolmogorov-Smirnov test were used on statistical analysis. The prevalence of OAS was 45.7%. The risk factors for OAS development were diabetes (p < 0.001), severity of nasal symptoms (p < 0.05) and severity of ocular symptoms (p < 0.001). In conclusion, the prevalence of OAS in the Split-Dalmatia County is very high. The risks factors for OAS in patients with SAR are diabetes and severity of nasal and ocular symptoms.