The aim of the study was to analyze the influence of mullein flower extract addition on the oxidative stability and antioxidant activity of cold-pressed oils with a high content of unsaturated fatty acids. The conducted research has shown that the addition of mullein flower extract increases the oxidative stability of oils, but its addition depends on the type of oil and should be selected experimentally. In rapeseed and linseed oil, the best stability was found for samples with 60 mg of extract/kg of oil, while in chia seed oil and hemp oil, it was found with 20 and 15 mg of extract/kg of oil, respectively. The hemp oil exhibited the highest antioxidant properties, as evidenced by an increase in the induction time at 90 °C from 12.11 h to 14.05 h. Additionally, the extract demonstrated a protective factor of 1.16. Oils (rapeseed, chia seed, linseed, and hempseed) without and with the addition of mullein extract (2–200 mg of extract/kg of oil) were analyzed for oxidative stability, phenolic compounds content, and antioxidant activity using DPPH• and ABTS•+ radicals. After the addition of the extract, the oils had from 363.25 to 401.24 mg GAE/100 g for rapeseed oil and chia seed oil, respectively. The antioxidant activity of the oils after the addition of the extract ranged from 102.8 to 221.7 and from 324.9 to 888.8 µM Trolox/kg for the DPPH and ABTS methods, respectively. The kinetics parameters were calculated based on the oils’ oxidative stability results. The extract increased the activation energy (Ea) and decreased the constant oxidation rate (k).

This study aimed to evaluate the quality of selected oils from the seeds of herbs and vegetables (basil, fenugreek, coriander, tomato, garden cress, parsley, and dill), especially their oxidative stability. The oils were tested for oxidation degree (acid value, peroxide value, p-anisidine value, TOTOX indicator, and specific extinction under ultraviolet light), colours, content of carotenoid and chlorophyll pigments, fatty acid composition, indicators of lipid nutritional quality, oxidative stability, and oxidation kinetics parameters (Rancimat). Principal component analysis was applied to identify a correlation between the oils’ quality parameters. The results of the fatty acid compositions show that basil oil was a good source of omega-3 fatty acids. Coriander seed oil was found to be the most resistant to oxidation, containing mainly monounsaturated fatty acids. The highest value of activation energy was calculated for fenugreek oil (94.18 kJ/mol), and the lowest was for dill seed oil (72.61 kJ/mol). However, basil oil was characterised by the highest constant reaction rate at 120 °C—3.0679 h−1. The colour determined by the L* parameter and the calculated oxidizability value had the most significant influence on the oxidation stability of the oils, and the correlation coefficients were r = −0.88 and 0.87, respectively.