The aim of the study was to evaluate the influence of preventive activities involving the use of a coccidiostatic product containing Origanum vulgare and Citrus spp. extracts on the slaughter value of lambs from a confined management system. The study material was a group of 222 mother ewes and 276 lambs of the Polish Lowland breed. During the whole experiment the animals were kept in a confined management system, indoors, with uniform environmental conditions. On the basis of the parasitological analysis the flock was divided into three groups similar in both the intensity and the extensity of parasitic infections: Group 1: a control group, whose diet was not supplemented (74 mother ewes, 87 lambs); Group 2: diet supplemented with OILIS SD, a product with coccidiostatic effect, from the third month of pregnancy to lambing the lambs were fed like the control group (73 mother ewes, 105 lambs); Group 3: diet supplemented with OILIS SD, a product with coccidiostatic effect, from the third month of pregnancy to lambing, the lambs receiving feed with OILIS SD starting from the 14th day of life (75 mother ewes, 84 lambs). Out of 276 lambs included in the experiment, 18 rams and 18 ewes representing all the observed groups were chosen for slaughter. The lambs were slaughtered around day 100 of their life (± 6 days). The observations showed that the animals from Group 3, in which the feed of both mother ewes and lambs was supplemented with a coccidiostatic product, were characterized by a higher slaughter weight: 30.80 kg, in comparison to 27.80 kg in Group 1 and 26.90 kg in Group 2, which has been confirmed as statistically significant (p ≤ 0.05). This corresponded to the achieved carcass weight. The highest average hot carcass weight (13.36 kg) was noted in Group 3, while the parameters for Group 1 (11.88 kg) and Group 2 (11.28 kg) were significantly lower (p ≤ 0.05). The conducted post-slaughter analysis clearly showed that the coccidiosis prevention measure consisting in a product containing the ether oils of Origanum vulgare (Lamiacea) and Citrus spp. (Citraceae) has a beneficial influence on the slaughter value of lambs, and results both in a higher carcass weight and the weights of specific cuts and tissues.