Resynthesized (RS) Brassica napus can be used to increase the genetic diversity of this important crop plant and to develop the heterotic gene pool required for successful hybrid breeding programmes. The level of heterosis in F1 hybrids depends on the individual performance of the parents and on the degree of genetic difference between them. However, RS forms obtained from crosses of B. rapa ssp. with B. oleracea ssp. possess many undesirable agronomic traits, such as low quality of seeds, low yield and seed oil content, high erucic acid level in the oil and high glucosinolate content in seed meal. Therefore, RS oilseed rape needs to be improved by crossing with natural double-low oilseed rape, leading to selected double-low quality semi-RS lines that can be used for breeding. In this study, we evaluated the seed yield potential of F1 hybrids derived from crosses between Ogura cytoplasmic male-sterility (CMS) lines and doubled haploid (DH) semi-RS lines with restorer gene in three locations in Poland. The genotype by environment interaction (GE interaction) and general combining ability (GCA) of the restorer and CMS line effects, as well as the effects of heterosis, were also assessed. The results of the study provide the first insights into the use of semi-RS lines as components for the development of new hybrid cultivars. Even the introduction of 50% of the RS oilseed rape genotype to natural restorer lines resulted in a marked heterosis effect, with seed yield ranging from 4.56% to 90.17% more than that of the better parent. The yield of the best hybrid amounted to 108.6% of the seed yield of the open-pollinated cultivar Monolit and 94.4% of that of the hybrid cultivar Arsenal. The best DH semi-RS line S1, which had a significantly positive GCA for seed yield, can be recommended as a possible parent for inclusion in breeding programmes aimed at developing new hybrid cultivars.