From what I have read, OPF came out of the original Open eBook format:
When I convert LIT files into HTML (for use on my eBookwise), I also end up with .opf files. I used to ignore them and then create ebooks out of the multiple HTML files -- with much grumbling as chapters always end up out of order. What I learned eventually was that I could drag the OPF file into the eBookwise Librarian program. It then uses the .opf file to create an IMP file that contains all the HTML files, plus cover images, etc. Doh!
I don't know which other programs can work the same magic with the OPF files, however.