Open Packaging Conventions

The Open Packaging Conventions (OPC) is a container-file technology initially created by Microsoft to store a combination of XML and non-XML files that together form a single entity such as an Open XML Paper Specification (OpenXPS) document. OPC-based file formats combine the advantages of leaving the independent file entities embedded in the document intact and resulting in much smaller files compared to normal use of XML.

The OPC is specified in Part 2 of the Office Open XML standards ISO/IEC 29500:2008 and ECMA-376.

The ISO/IEC 29500-2:2008 specification and the second edition of ECMA-376 makes a normative reference to PKWARE, Inc.'s .ZIP File Format Specification version 6.2.0 (2004), and supplements it with a normative set of clarifications. Note: The older first edition of ECMA-376 makes an informative (i.e., non-normative) reference to the newer PKWARE Inc's ".ZIP File Format Specification" version 6.2.1 (2005). The ZIP format is not specified by any international standard, but has widespread community and developer acceptance.

Microsoft submitted a draft in 2006 to the Internet Engineering Task Force for a "pack" URI Scheme (pack://) to be used for URI references to OPC-based packages. The draft expired in 2009, the specified syntax is incompatible with the Internet Standard for URI schemes (STD 66, RFC 3986). The scheme is now listed as historical.

Both the XML Paper Specification (XPS) and Office Open XML (OOXML) use Open Packaging Conventions (OPC), which provide a profile of the common ZIP format. In addition to data and document content in XML markup, files in the ZIP package can include other text and binary files in formats such as PNG, BMP, AVI, PDF, RTF, or even an already packaged ODF file. OPC also defines some naming conventions and an indirection method to allow position independence of binary and XML files in the ZIP archive.