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The Once-Only hub.Unlocking the full potential of the Single Market.

Welcome to the Once-Only Hub, a single environment providing reliable information and services relating to the Once-Only Technical System.

The Once-Only Technical System lets EU public authorities securely exchange official documents and data at the request of citizens and businesses as part of cross-border administrative procedures, e.g., when registering an address or vehicle when moving abroad. This innovative system eliminates the complicated manual search, mapping, and fetching of data between Member State authorities, improving efficiency, making people's lives easier, and reducing the costs of mobility in the Single Market.

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Do you need to connect to theOnce-Only Technical System?

Find out if your authority is in scope of the Single Digital Gateway Regulation and the Once-Only Technical System.

Introduction to the Once-Only Technical System (OOTS)

The Once-Only Technical System (OOTS) is a framework designed to facilitate data sharing across borders. It will allow citizens and businesses to request official documents, or "evidence," needed for administrative procedures, thus making it easier for them to study, work, live, or retire when moving between EU countries. The system’s decentralised architecture connects public authorities, so they can exchange evidence at the citizen's or business' request.

Discover the 21 procedures covered by the SDG Regulation in the scope of Once-Only Technical System and how EU citizens and businesses can benefit from Once-Only Technical System.

Discover the Once-Only Technical System overview

Once-Only Technical System - user journey for cross-border procedures

The user-journey refers to the steps anyone, a citizen or business, must take when using the Once-Only Technical System with a mind to the overall experience the user has.

Discover the OOTS user journey

Once-Only Technical System architecture

The Once-Only Technical System architecture adopts a decentralised approach, integrating and connecting systems through an interoperability layer using reusable "Building Blocks".

Learn about the OOTS architecture

OOTS Guidance Presentation

The Guidance Presentation provides a useful introduction for teams across the EU working on the implementation of the Once-Only Technical System.

Download OOTS Guidance Presentation

Once-Only Technical System Projectathons

Since 2023, the Commission has been organising Once-Only Technical System Projectathons. These are three-day events where national teams connect various systems and perform a marathon of peer-to-peer interoperability and compliance tests in a structured environment.

Discover more details about Projectathons and explore the steps you need to take to actively participate in the upcoming Projectathons.

Go to Projectathons

Projectathon #6

The June 2025 Projectathon supported Member States that were (nearly) production-ready to enable end-to-end transactions, while assisting those catching up to enhance their technical readiness or retest the updates to their systems. This Projectathon also supported the onboarding of competent authorities, in particular those in the population, education and business domains as well as those interested in testing and/or close to going live.

11 - 12 June, 2025 | Brussels

Go to Projectathon #6

Projectathon #5

The November 2024 Projectathon was a thematic event with a focus on Vehicle Registries and Business-related procedures, targeting this specific set of Competent Authorities.

20 - 22 November, 2024 | Brussels

Go to Projectathon #5

Common and supporting services

To provide comprehensive support and proper exchange of evidence between Data Services and Online Procedure Portals, the Once-Only Technical System uses Once-Only common services and additional supporting services.

Go to common & supporting services

Technical design documentation

The technical and operational specifications of the Once-Only Technical System, are complemented by detailed, non-binding technical design documents jointly prepared by the Commission and Member States.

Go to Technical design documentation

Latest news

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Collaborate with us

SDG Once-Only Collaborative Space

In the context of the Once-Only Technical System, the mission of this group is to coordinate the implementation work around the setup of the Once-Only Technical System. The members of this group are the Member States (and other public entities). This groups is only accessible by representatives of the Member States (and other public entities).

Restricted space

Sub-groups

Six sub-groups have been set-up to examine specific topics related to the Once-Only Technical System and are expected to report to the Single Digital Gateway Coordination Group. They enable the participation of all Member States in the discussion and ensure the work done by Member State experts is monitored. The sub-groups can only be joined by experts officially nominated by their National Coordinators.

Restricted space

 

OOTS Projectathon collaborative space

The objective of this space is to encourage dialogue and exchange of ideas between stakeholders in an open and transparent environment. It will mainly serve to promote the Implementers Café series in preparation of the Projectathons. Space users should be implementers of the Once-Only Technical System.

Service desk

The Service Desk helps users improve the performance, continuity and reliability of their Once-Only Technical System implementations.

Contact the Service Desk

Once-Only Technical System in use

Projectathons

25Member States

Number of Member States participating in the last OOTS Projectathon.

Common Services

27Member States

Number of Member States connected to Commission-provided Common Services.

Testing Services

26Member States

Number of Member States connected to Testing Services.

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