Plant-derived preparations, particularly essential oils (EOs), are gaining attention as possible tools for combating infections caused by gastrointestinal nematodes (GINs). This research explored the in vitro efficiency of 11 EOs and one binary blend of isolated EO constituents, along with the in vivo antiparasitic effects of two EO-based treatments. Four nematode genera—Haemonchus, Trichostrongylus, Teladorsagia, and Chabertia—were identified through coproculture analysis. The in vitro egg hatch assay (EHT) was carried out at six graded concentrations (50, 12.5, 3.125, 0.781, 0.195, and 0.049 mg/mL) for every EO sample. For the in vivo faecal egg count reduction test (FECRT), each oil was dissolved in sunflower oil and administered orally at 100 mg/kg. Based on EHT findings, Origanum vulgare, Foeniculum vulgare, Satureja montana, Satureja hortensis, and two Thymus vulgaris chemotypes showed the highest inhibition rates. Their primary molecules—carvacrol, thymol, anethol, p-cymene, and γ-terpinene—appeared to play a key role in nematode suppression. In FECRT trials, T. vulgaris EO type 1 and the linalool: estragole blend achieved roughly 25% mean FECR, suggesting modest in vivo potential. Overall, these outcomes point to the possible use of selected EOs as natural anthelmintics in sheep management, though more extended in vivo investigations are warranted.
