Even though modern analytical chemistry has developed a methodology enabling evaluation of the presence of OTC in milk, data regarding its concentration in the material collected from lactating sows are missing. Therefore, this paper was intended to provide new data on the transmission of OTC and its epimer, 4-epi-oxytetracycline (4-epi-OTC), in the milk of lactating sows after a singular intramuscular administration of a long-acting form of the antibiotic. The determination of OTC and 4-epi-OTC was carried out using ultrahigh-performance liquid chromatography with mass spectrometry (UHPLC–MS/MS). The highest average concentration of antibiotic (1132.2 µgL−1) was observed in samples collected 1 day after the administration of the drug. The average OTC level at day 3 was 358 µgL−1. The average concentration of the antibiotic found on the 21st day was 12.3 µgL−1. The highest average concentration of 4-epi-OTC—i.e., 54 µgL−1—was noted 1 day after the administration. Amongst samples collected at day 3, the average level of the substance in question was 26.4 µgL−1. The average value observed at day 21 was 1.5 µgL−1. Our results indicated considerable OTC and 4-epi-OTC transmission into the milk of lactating sows.