Summary
Behind the OMOP model stands an open international community: OHDSI (pronounced “Odyssey”). Founded in 2014, it brings together more than 4,200 collaborators across 83 countries, and a network of databases covering roughly 810 million patient records. This article explores how it works: organisation, federated research, landmark studies, major institutional projects — and how to join in.
From project to community
In the previous article, we saw that the OMOP project came to a close shortly before OHDSI was founded in 2014, after proving that analyses could be compared across heterogeneous databases. But the project itself had to end — it was a time-limited research programme.
In 2014, the researchers and institutions who had taken part founded OHDSI (Observational Health Data Sciences and Informatics), an open international community whose coordinating centre is based at Columbia University (New York).
Unlike the OMOP project, which had a defined start and end, OHDSI is designed as a lasting community, open to all, with no membership fee. Its official mission, as stated in the Book of OHDSI:
The OHDSI mission
“To improve health by empowering a community to collaboratively generate the evidence that promotes better health decisions and better care.”
An open community
Six principles at the heart of the community, set out in the Book of OHDSI (section 1.3.3):
Innovation
”Observational research is a field which will benefit greatly from disruptive thinking. We actively seek and encourage fresh methodological approaches in our work.”
Reproducibility
”Accurate, reproducible, and well-calibrated evidence is necessary for health improvement.”
Community
”Everyone is welcome to actively participate in OHDSI, whether you are a patient, a health professional, a researcher, or someone who simply believes in our cause.”
Collaboration
”We work collectively to prioritize and address the real-world needs of our community’s participants.”
Openness
”We strive to make all our community’s proceeds open and publicly accessible, including the methods, tools and the evidence that we generate.”
Beneficence
”We seek to protect the rights of individuals and organizations within our community at all times.”
In practice, this translates into:
- All tool source code lives on GitHub, under the Apache 2.0 licence.
- Reference documentation is free — the Book of OHDSI is freely available and is the official source on the model, methods and tools.
- Study protocols and results are deposited publicly as soon as a network study is completed.
The federated research model
One of OHDSI’s most innovative aspects is its distributed research model (Book of OHDSI, section 1.4). Here is how it works:
Data stays local
Each institution keeps its data in its own environment — patient-level data never leaves the hospital.
Protocols and code travel
A study protocol and analysis code are shared publicly with partners who wish to take part.
Each partner runs the code locally
The analysis runs on each partner’s data, in their own secure environment. No remote access to data is required.
Only aggregated results flow back
Only the aggregated results (never individual data) are returned for the international synthesis.
This model solves three problems at once:
- Data sovereignty: hospitals retain full control over their data.
- Patient privacy: no individual data ever circulates.
- Scale: studies can be run on hundreds of millions of patients without building a giant central database (which would be both technically and legally impossible).
A global footprint
OHDSI today spans every continent:
- 4,200+ collaborators
- 83 countries represented
- ~810 million patient records in the data network
These figures come from OHDSI’s 2025 annual report and are published every year by the community itself.
Regional chapters
To stay close to the ground, OHDSI is organised into regional chapters, listed on the official site: Europe, Africa, Australia, China, Korea, India, Japan, Singapore, Taiwan. The Europe chapter is based at Erasmus MC (Rotterdam) and runs a yearly symposium.
The French-speaking community is still emerging, but several French teams are active contributors — notably through the Plateforme de Données de Santé (PDS, formerly Health Data Hub) and several university hospitals.
How the community lives day to day
OHDSI comes alive day to day through several channels open to all:
The OHDSI forums
The central place for exchanges. Technical questions, study proposals, lessons learned.
Weekly community calls
Every Tuesday: research talks, tool demos, methodological discussions. Open to all, recorded.
Workgroups
More than 20 thematic workgroups: ATLAS/WebAPI, Common Data Model, NLP, Oncology, Medical Imaging, Network Data Quality…
Annual symposiums
The main global gathering (United States) and its European counterpart. Plenary sessions, posters, hands-on tutorials.
On top of these come the study-a-thons and hack-a-thons: intensive multi-day sessions during which the community collectively produces protocols, code and results.
Major institutional projects
The momentum is accelerating: regulatory agencies, national programmes and European consortia now rely on OHDSI and the OMOP model to structure their data networks.
In Europe
- EHDEN (European Health Data & Evidence Network) — funded by IMI2, EHDEN supported the harmonisation of a large number of European databases to the OMOP CDM and built a training offering (the future EHDEN Academy). The project gave rise to the EHDEN Foundation, which continues its activities.
- DARWIN EU — data network led by the European Medicines Agency (EMA) to produce evidence from routine clinical care (rather than clinical trials) to support medicines regulation. Built around the OMOP model, it is coordinated from Rotterdam and fully operational since 2024.
- INDICATE Europe — launched in late 2024 and co-funded by the EU’s Digital Europe programme, INDICATE is building a federated infrastructure for cross-border sharing of intensive-care data. OMOP is part of its training and standardisation programme.
- The EHDS (European Health Data Space) — EU regulation that came into force on 26 March 2025, establishing a common framework for the primary and secondary use of health data across the EU. In the evaluation of interoperability standards conducted by the TEHDAS joint action in preparation for the EHDS, OMOP-CDM received the highest score.
In the United States
- All of Us (NIH) — a precision medicine programme whose EHR data is made available in the OMOP CDM format.
- CHoRUS (Collaborative Hospital Repository Uniting Standards for Equitable AI) — a US project building a multi-modal hospital repository (EHR, waveforms, imaging, free text) ready for AI, with systematic conversion to OMOP CDM in its standard pipeline.
In France
- Plateforme de Données de Santé (PDS, formerly Health Data Hub) — the public-interest grouping that coordinates access to health data in France has led work to convert the SNDS (Système National des Données de Santé) to OMOP.
- Several French university hospitals have adopted or are piloting the OMOP format for their data warehouses.
Other national initiatives exist around the world, driven by OHDSI’s regional chapters.
Joining the community
To get started, here is a concrete six-step path:
- Read the Book of OHDSI — the official, free reference document.
- Sign up to the forums — the welcome is friendly, including for newcomers.
- Attend a community call — every Tuesday, the schedule is public.
- Take the EHDEN Academy — free online training on OMOP, ETL, ATLAS, vocabularies. The best practical entry point.
- Explore the workgroups — find the one matching your domain and join the meetings.
- Follow OHDSI on GitHub — all the tools, study protocols and example code are published there.
- OHDSI (pronounced "Odyssey") emerged in 2014 from the community that had formed around the OMOP project, and today coordinates a network of 4,200+ collaborators in 83 countries.
- At the heart of OHDSI: a federated model in which patient-level data does not leave its institution — only aggregated results are shared, potentially at the scale of hundreds of millions of patients.
- The community lives through the forums, the weekly community calls, the workgroups, the annual symposiums and the study-a-thons.
- Institutional adoption is accelerating: EMA (DARWIN EU), NIH (All of Us), INDICATE, EHDS, Plateforme de Données de Santé, and other national initiatives.
- The best entry points to learn: the Book of OHDSI and the EHDEN Academy, both free.