From Ledet Training Wiki
.opf is the extensioin used for epublishing documents for the manifest file. A .opf file is 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 the OPF Specification.
.opf files are part of the .epub standard but they are also created by applications such as Calibre for storing metadata on other books in the library such as pdf files.