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Natural polyphenols may be very effective lipid antioxidants; however, the studies pertaining to specific commodities, such as refined olive kernel oil (OKO), are extremely limited, and thus there is a lack of relevant information. To provide coverage for this gap in the literature, this study was performed with the view of testing structurally related natural hydroxycinnamates, namely caffeic acid (CA), ferulic acid (FA), and p-coumaric acid (CouA), as effective OKO antioxidants, and establishing concentration–activity relationships. First, a ranking concerning the antioxidant potency of the three polyphenols was attempted using the radical probe DPPH, and then the compounds were assayed using OKO as the lipid substrate. Employing a concentration–activity model, the potency of the three polyphenols to stabilize OKO, based on Rancimat measurements, was CA > CouA ≈ FA. A further simulated long-term trial revealed that all three polyphenols may be equally effective in inhibiting peroxide onset over a period of 40 days, but the determination of thiobarbituric acid reactive substances (TBARS) formation evidenced that FA may be a more efficient antioxidant. The outcome was that all three polyphenols tested may behave as very effective agents against oxidative OKO rancidity, and the prospect of producing such compounds from agri-food wastes may provide unprecedented opportunities for replacing the controversial synthetic antioxidants.