Olive leaf and olive mill wastewater are wastes which are produced in olive industry and can be used to obtain value-added products such as polyphenols. In this work, extracts of polyphenols from olive mill wastewater and olive leaf in an aqueous medium are obtained and their composition in polyphenols were determined and compared with those obtained using the conventional extract agent (methanol−water). The antioxidant capacity of the aqueous extracts of olive mill wastewater and olive leaf were also analysed. It was observed that the olive mill wastewater extracted in water provides a similar content of phenolic compounds in the final extract compared to extraction with methanol−water, with the main polyphenols being hydrotyrosol and tyrosol (20.1 and 6.61 mg gdw−1). In the case of leaf extraction, the methanol-free extract (recovery in water after extraction with methanol−water) had a lower total phenol content compared to the methanolic extract, with the main polyphenol being oleuropein (22.73 and 9.05 mg gdw−1, for the methanol and methanol-free extract, respectively). However, both extracts obtained in aqueous solution present a similar antioxidant capacity at very diluted concentrations of the original extract, with IC50 values (half-maximal antioxidant concentration) of about 20 mg TS L−1. The antioxidant capacity of the extract in aqueous solution facilitates its application as an antioxidant in biological systems, like animal food, where the use of extracts based on organic solvents, like methanol, are not suitable.