The Open Container Initiative (OCI) is, in its own words, “a lightweight, open governance structure (project), formed under the auspices of the Linux Foundation, for the express purpose of creating open industry standards around container formats and runtimes.” It was launched on June 22, 2015, by Docker, CoreOS, and other leaders in the container industry.
The OCI exists because, by 2015, containers risked fragmenting. Docker had popularized the format, but rival efforts and the threat of incompatible image and runtime formats raised the prospect that a container built with one tool might not run under another. Standardizing the format under neutral governance kept the ecosystem portable.
The OCI maintains three core specifications. The Runtime Specification (runtime-spec) defines how to run a filesystem bundle as a container. The Image Specification (image-spec) defines how a container image is structured. The Distribution Specification (distribution-spec) standardizes the API for distributing images between registries and clients; it reached v1.0 in 2020. Together they describe the full path of downloading an image, unpacking it into a runtime bundle, and executing it.
Because Docker, Podman, containerd, Kubernetes, and the registries they pull from all conform to these specifications, an OCI image built once can run essentially anywhere. The OCI is the quiet standards body that makes “build once, run anywhere” true for containers, rather than a promise tied to any single vendor’s tooling.