opc-diag is a command-line application for exploring Microsoft Word, Excel, and PowerPoint files from Office 2007 and later. Also known as Office Open XML, the structure of these files adheres to the Open Packaging Convention (OPC), specified by ISO/IEC 29500.

opc-diag provides the opc command, which allows OPC files to be browsed, diff-ed, extracted, repackaged, and parts from one to be substituted into another.

Its primary use is by developers of software that generates and/or manipulates Microsoft Office documents.

A typical use would be diff-ing a Word file from before and after an operation, say inserting a paragraph, to identify the specific changes Word made to the XML. This is handy when one is developing software to do the same without Word's help:

$ opc diff before.docx after.docx

Another main use is to diagnose an issue causing an Office document to not load cleanly, typically because the software that generated it has a bug. These problems can be tedious and often difficult to diagnose without tools like opc-diag, and were the primary motivation for developing it.

More information is available in the opc-diag documentation