Widespread interpopulation hybridization can lead to outbreeding depression and homogenization of the wider gene pool. To benefit from local adaptation and to avoid interpopulation hybridization, recommendations and policies in European forestry are focused on the use of native and locally adapted seed sources in afforestation and reforestation. Defining a local or native provenance is predicated on knowledge about original seed sources. In places such as Denmark, where provenances of oak have been imported from many parts of Europe, lacking historical records can make this difficult. Advances in genomics have improved the resolution of detectable genetic differences between populations, giving us the opportunity to determine the source of provenances in the absence of written records. We compare whole genomes of 109 pedunculate oak (Quercus robur) samples belonging to provenances both native and nonnative to Denmark for the purposes of evaluating the suitability of these provenances for future use as native Danish seed sources. We analyze population structure and differentiation, admixture, and measures of genetic variation, and address the role of Q. robur in current and past Danish forestry, including the widespread introduction of foreign genetic material. We could not with certainty determine whether the studied provenances are native, but find that there are varying levels of admixture between provenances in the Danish Q. robur population. The Danish Q. robur population overall appears to be cohesive, although it acts as a continuous population rather than a group that is fully differentiated from foreign provenances planted nearby. We do not detect introgression from Quercus petraea. We conclude that the studied provenances represent diverse seed sources with little signs of inbreeding or previous bottleneck events and that they are therefore suitable for future use in Danish forestry.