The objectives of the study was to investigate and compare the characteristics of oak advance−growth of various age present under Scots pine canopy and growing on the mesic mixed coniferous (BMśw) or mesic mixed deciduous (LMśw) forest site types. The study was conducted in Kolumna Forest District (central Poland) in 21 Scots pine stands ca. 90 years of age, and with a well−developed oak advance−growth. The study sites were divided into 4 groups depending on the age of oak: 15, 20, 25 or 30−years−old. The empirical data concerned trees frequency (per unit area), average breast height diameter, height and basal area. The dependence of these characteristics on type of the main stand and advance−growth was then determined by statistical analysis. Above all, an attempt was made to determine the impact of forest site type conditions on the characteristics of oaks advance−growth. Results show that oaks are present at the density that makes the possibility of the advance−growth development into the stand real. Trees in the upper storey are found to exert a great influence on the silvicultural value of advance−growth trees. In particular, the density of trees in the main stand is a feature found to affect greatly the layer of oak advance−growth.