Explore the critical issue of online sexism and learn about an innovative open-source activism project in this EuroPython 2019 conference talk. Delve into the global problem of online sexual harassment targeting women and discover how the browser extension "Opt Out" aims to combat this issue. Gain insights into the challenges of building an open-source activism project led by the affected community. Examine current research, community engagement results, and the origins of the idea. Investigate the complexities of applying natural language processing (NLP) to detect online harassment, and understand the balance between cutting-edge data science and web development in an open-source community. Learn about the project's technical infrastructure, including browser extensions, sentiment dashboards, and web applications. Explore ethical considerations, collaboration with social media platforms, and the vision for the future of this important initiative.