Oat is an important crop in Nordic countries both for feed and human consumption. Maintaining a high level of genetic diversity is essential for both breeding and agronomy. A panel of 94 oat accessions was used in this study, including 24 museum accessions over 100- to 120-year old and 70 genebank accessions from mainly Nordic countries and Germany, covering different breeding periods. Sixty-one polymorphic SSR, 201 AFLP and 1056 DArT markers were used to evaluate the past and present genetic diversity of the Nordic gene pool. Norwegian accessions showed the highest diversity, followed by Swedish and Finnish, with German accessions the least diverse. In addition, the Nordic accessions appeared to be highly interrelated and distinct from the German, reflecting a frequent germplasm exchange and interbreeding among Nordic countries. A significant loss of diversity happened at the transition from landraces and old cultivars to modern cultivars. Modern oat originated from only a segment of the landraces and left the remainder, especially black oat, unused. However, no significant overall diversity reduction was found during modern breeding periods, although fluctuation of diversity indices was observed. The narrow genetic basis of the modern Nordic gene pool calls for increasing genetic diversity through cultivar introduction and prebreeding based on neglected sources like the Nordic black oat.