A field study was conducted at the Certified Organic Farm in Poland over the period 2014–2016. This study evaluated weed infestation and seed yield of the lentil varieties ‘Tina’ and ‘Anita’, as well as of a mixture of these two varieties, sole cropped and row intercropped with naked oats as a supporting crop. Additionally, lentil was sown at a different row spacing of 20 and 25 cm. The lentil variety ‘Anita’ produced 25.3% higher yields than var. ‘Tina’. Weight of 1000 seed, number of pods per plant, and first pod height did not differ significantly in the treatments with the lentil varieties. The lentil seed yield obtained in the treatments with a supporting crop was lower by 9.4% compared with the sole cropped plots. In turn, the 1000 seed weight was 3.9% higher in the treatment where a supporting crop was used. The presence of oats as a supporting crop in lentil crop allowed crop competitiveness against weeds to be increased significantly, thus reducing their total number and dry weight by 5.3% and 30.5%, respectively. Sowing lentil at different row spacings did not have a significant effect on seed yield and weed infestation in crop. The greatest diversity of weed species was found in the treatments where the mixture of the lentil varieties was sown and in the treatment with the smaller row spacing. Intercropping of lentil with oats resulted in reduced occurrence of monocotyledonous weeds such as Echinochloa crus-galli and Elymus repens. The presence of oat as a supporting crop can effectively reduce the pressure from weeds without significantly reducing lentil cultivar yield in organic farming.