Olive mill wastewater (OMWW), a by-product of olive oil production, poses severe environmental challenges due to its toxicity, primarily caused by its high organic load and phenolic compounds, along with organic acids, lipids, and heavy metals. These components contribute to its elevated chemical and biological oxygen demand, making OMWW a persistent pollutant that necessitates urgent and effective treatment strategies. The ecological risks, including water contamination, soil degradation, and biodiversity loss, underscore the need for sustainable management approaches. This review explores the composition and toxicity of OMWW, examining advanced treatment technologies, e.g., bioremediation, membrane filtration, advanced oxidation processes, and integrated systems that enhance efficiency while minimizing environmental impact. In addition, this study investigates the potential for OMWW valorization as a rich source of polyphenols with antioxidant, antimicrobial, and anti-inflammatory properties. These bioactive compounds have significant economic value in industries such as pharmaceuticals, cosmetics, and functional foods. By evaluating sustainable extraction techniques and integrating advanced treatments with economic valorization, OMWW can be transformed from an environmental pollutant into a valuable resource. Such integrated approaches support a circular economy within the olive oil industry, reducing its ecological footprint and fostering sustainable development.