Open Packaging Format

File extension OPF in order for electronic-book technology to achieve widespread success in the marketplace, Reading Systems need to have convenient access to a large number and variety of titles. File extension OPF, another related specification, the Open Publication Structure (OPS) Specification, File extension OPF describes a standard for representing the content of electronic publications and is meant to reduce barriers to the proliferation of content. Specifically, the specification is intended to:
Give publication tool providers and content providers (e.g. publishers, authors, and others who have content to be displayed) minimal and common guidelines that ensure fidelity, accuracy, accessibility, and adequate presentation of electronic content over various Reading Systems; and
Build on established content format standards; and
Define a standard means of content description in order for electronic books to move smoothly through the distribution chain.

Windows cannot open OPF files?
If Windows is unable to open file extension .OPF you may have registry related errors or may not have the appropriate program installed. We strongly recommend that you run a free registry scan.
Appropriate program:
OPF is a file extension associated with FlipAlbum OPF is a file extension associated with . Incorrect associations are the cause of many file extension errors. Scan your system to prevent, identify and repair OPF file extension association errors.

This document, the Open Packaging Format (OPF) Specification, defines the mechanism by which the various components of an OPS publication are tied together and provides additional structure and semantics to the electronic publication. Specifically, OPF:
Describes and references all components of the electronic publication (e.g. markup files, images, navigation structures).
Provides publication-level metadata.
Specifies the linear reading-order of the publication.
Provides fallback information to use when unsupported extensions to OPS are employed.
Provides a mechanism to specify a declarative table of contents (the NCX).
This OPF specification is separate from the OPS markup specification to modularize the described packaging methodology and the described content. This will help facilitate the use of this packaging technology by other standards bodies (e.g. DAISY) in non-OPS contexts.
A third specification, the OEBPS Container Format (OCF) Specification, defines the standard mechanism by which all components of an electronic publication can be packaged together into a single file for transmission, delivery and archival.

one of definition from file extension opf is

OPF Package Document
An XML Document that describes an OPS Publication and references all files used by the OPS Publication that are not part of the OPF Package Document itself. It identifies all other files in the Publication and provides descriptive information about them. The OPF Package Document is defined by this specification and is valid to the OPF Package Schema defined herein.
The "root file" of the OPF Package Document should use the .opf extension. This XML file may refer to other XML files via XML’s general entity mechanism, but those files must not use the .opf file extension. This construction could be used to simplify the creation of OPF Package Documents for very large Publications. However, the most common case is for the OPF Package Document to be a single XML file using the .opf extension.