Several in vitro and in vivo studies have shown that olive phenols exert potent biological activities including, but not limited to, antioxidant actions. These activities are shared by phenols found in olives, olive oil, and olive mill wastewater (OMWW). The aim of this study was to investigate whether a commercially available OMWW preparation could influence some parameters of oxidative status in healthy human volunteers. Ninety-eight healthy subjects with normal body weight were recruited, and 5 mL of blood was drawn from their antecubital vein after an overnight fast of at least 12 h. After this, subjects were asked to ingest 2 mL of a commercially available OMWW preparation. Another 5 mL of blood was drawn 1 h after ingestion of the preparation. Plasma antioxidant capacity and total and reduced glutathione were measured. No difference in plasma antioxidant capacity was observed between baseline and 1 h after the ingestion of the extract. Conversely, a significant increase in total plasma glutathione concentration was measured. This increase involved both the reduced and oxidized forms of glutathione; hence, their ratio was unaffected by the treatment. The observed effects of OMWW on glutathione levels might be governed by the antioxidant response element (ARE)-mediated increase in phase II enzyme expression, including that of gamma-glutamylcysteine ligase and glutathione synthetase. Future studies on groups of individuals who may benefit from an increase in their glutathione levels, for example, the elderly, will further elucidate the biological activities of this formulation.