Background/Objectives: The health-protective properties of olive oil phenolics, including their potential chemopreventive and anticancer effects, have attracted considerable scientific interest. However, their in vivo efficacy and mechanisms of action remain insufficiently understood. Recent advances in extraction and purification technologies have enabled large-scale production of highly purified olive oil phenols and phenolic-rich extracts, facilitating translational research. Methods: Herein, the antitumor efficacy of isolated olive oil phenols and phenolic-rich formulations was investigated in an MDA-MB-231 triple-negative breast cancer (TNBC) xenograft model. Results: Intraperitoneal administration of oleocanthal (OLC), oleuropein aglycone (OleA) or their combination reduced endpoint tumor burden, with OLC exhibiting the most pronounced effect. Oral administration of total olive oil phenolics (OOPs) achieved comparable efficacy. Pre-treatment initiated before tumor cell implantation conferred the greatest protection, consistent with a prophylactic mode of action. In contrast, delayed intervention displayed diminished or no antitumor benefit. Phenolic-rich extra virgin olive oil likewise showed an inhibition trend in tumor progression. Mechanistically, OOPs attenuated plasma protein oxidation, modulated proteasome mediated proteolysis, and reduced γH2AX levels in vivo. Furthermore, OOPs negatively affected the MDA-MB-231 cell migration in a concentration-dependent manner in vitro. Conclusions: Collectively, these findings are consistent with antitumor activities of olive oil phenolics via multiple mechanisms and support their further investigation as prophylactic agents in TNBC and as nutraceuticals.