OCI Image Specification

The OCI Image Specification is an open standard defining the format and layout of container images, maintained by the Open Container Initiative (OCI). Founded in 2015 by Docker, CoreOS, and other industry leaders under the Linux Foundation, the OCI develops open standards for container formats and runtime. The image specification defines how container images are structured, enabling interoperability between different container engines and registries. With over 3,500 stars as of 2026, the specification is implemented by Docker, containerd, Podman, CRI-O, Kubernetes, and every major container platform. Key features include: image manifest (a JSON document describing the contents of a container image including layers, media types, and configuration), image index (a higher-level manifest supporting multi-architecture images, listing manifests for different platforms like amd64, arm64, and s390x), image layout (the on-disk directory structure for storing OCI images in a registry or local filesystem), layer format (defining how filesystem changes are represented as compressed tar archives with specific media types), configuration format (describing the image configuration including environment variables, entry points, volumes, exposed ports, and resource limits), digest-based content addressing (using SHA-256 digests to uniquely identify and verify image content, enabling deduplication and integrity verification), media types (standardized MIME types for manifests, configs, and layers, enabling extensibility for future formats), and annotation system (key-value metadata attached to manifests and configs for build provenance, source control information, and vendor-specific metadata).