Specifications
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 2nd 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 1st 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 has 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.
Read more about this topic: Open Packaging Conventions