changed the extension of an .omod to .7z and it had unreadable files in it (readme could be added/opened with Notepad but had a few wierd characters in it), a file named "data" that wasn't a folder, couldn't be changed to .7z to open.
There is no point in this extraction stuff. If you already have an archive version of this mod (the one you are trying to compare the OMOD's contents too) just use the 'Info' option from the context menu in OBMM to look through the archive. If you really want to convert the OMOD into an archive, choose 'Convert to archive' from that same menu. However, that is not at all necessary for your purposes.
What mod is this that you are trying to check, anyway?