The aim of this study was to investigate the effect of mullein flower extract on the oxidative stability of
linseed oil. The theoretical part discussed the composition, properties, and applications of linseed oil and
mullein. The lipid oxidation process and techniques for enriching oils with antioxidant compounds,
including the use of ultrasound, were described. In the experimental part, the ultrasound-assisted extraction
of antioxidant compounds from mullein flowers was optimized, with antioxidant activity (AA) used as the
optimization parameter. The optimized extract was added to linseed oil, determining the optimal addition
level at 5%. Both fresh linseed oil and the oil enriched with the extract were subjected to safety analysis,
and their oxidative stability and bioactive compound content were determined. The results confirmed the
effectiveness of mullein extract as a natural antioxidant, with linseed oil stability increasing on average by
26%. Moreover, the oil with the extract showed a higher degree of hydrolysis and a higher level of
secondary oxidation products. The addition of the extract increased the content of bioactive compounds in
linseed oil, especially phenolics, which increased threefold to values ranging from 311.49 to 430.41 mg
GAE/100 g of oil. The induction time of the oil was strongly positively correlated with the phenolic content
(r = 0.97), anisidine value (r = 0.95), and antioxidant activity of the oil’s hydrophilic fraction (r = 0.93).