This Policy Brief explores the EU Once-Only Technical System (OOTS), a key tool to implement the Once-Only Principle (OOP) and simplify cross-border administrative procedures for citizens and businesses. Backed by the Single Digital Gateway Regulation, the OOTS enables public authorities in different Member States to securely exchange official documents and data directly, avoiding the repeated submission of the same information. It is built on a decentralised architecture using existing national systems, supported by interoperability enablers like eID and eDelivery, and common services provided by the European Commission. The document outlines the challenges the OOTS addresses, such as fragmented procedures and paper-based processes, and highlights its benefits, including reduced administrative burden, enhanced trust, and economic savings. It details the technical components, the collaborative implementation process involving the Commission, Member States, and local authorities, and the impact on citizens, businesses, and administrations. The brief concludes with lessons learned, stressing interoperability, user-centric design, and stakeholder coordination, and provides practical recommendations for public administrations to begin their OOTS integration journey.