According to what I have just looked up, .opf is only a metadata file and does not contain the actual book.
Certainly, all the .opf files I have were created by Calibre when looking at (nonDRM) .epub books.
How big are your .opf files?
Do you have .epub files areound for those books as well? Try opening those with ADE.
Some filetypes (eg .acsm) won't open in ADE using the file open (ctrl-O) option, but will open using drag drop from Explorer onto ADE.
You could try drag-dropping your .opf files ~~~ drag-dropping mine doesn't work.