While the OMOP CDM supports a diverse set of use cases and data types, we acknowledge that not all can be represented in the CDM in its current form. Based on your use case it may be necessary to modify just your database, create an expansion, or suggest changes to the model that will be incorporated into the next official version. This document describes the types of additions that can be made to the CDM and how they should be utilized by the community.

Site - Specific Add on

It is entirely within your rights as a data owner or holder to create additional columns or even tables in your own instance of your CDM that is specific to your site. However, they are not supported or recognized by OHDSI tools and not part of the canonical OMOP CDM. The Add on column or table is used for a site specific use case or data element. Examples include: privacy flags, source table identifiers or other data elements. See this poster for background information and an implementation guide.


These are typically sets of tables created by working groups to model a specific type of data. The idea is that these tables follow the general conventions of the OMOP CDM but are not part of the canonical data model and therefore do not fall under the purview of the CDM working group. They are developed using OMOP conventions applied to domain specific datasets. It is possible they could become part of the CDM in the future (like the episode and episode_event tables) but that is not a given. If using an expansion/extension with your data they are invisible to the tools. They can be used as part of network research but you would need to ensure that all participating databases are using the extension/expansion before writing a study package that incorporates the tables.

Examples include:

Change to Canonical CDM

Please see the full process for how to request changes to the current version of the OMOP CDM. These change requests MUST be backed by network uses cases and MUST be approved by the CDM working group. Overall these are typically small changes or additions that make the CDM more usable from a network perspective or fulfill a clearly demonstrated need. Good examples are the recently ratified NOTE_NLP updates and presentation by the vocabulary team to add value_source_concept_id to the measurement and observation tables.