Shredded, healthy, fallen oak leaves and compost are effective, natural, spring-applied, time-release, water-soluble fertilizers for oak trees. Oaks favor acidic soils so they like the acidic fertilizer which can be formed from acidic compost and their own acidic leaf litter.
But the actual choice depends upon a soil analysis. Alkaline soils above pH7.0 make iron and manganese unavailable even if they're present in sufficient amounts in the soil. Acidic soils below 5.5 may make aluminum a toxic nutrient in the soil. A store-bought, organic fertilizer higher in nitrogen, lacking in phosphorus, and lower in potassium tends to solve the previously mentioned deficiencies, along with lime treatments for acidic soils and sulphur for alkaline.