OrCAD's file formats DSN and OLB use Microsoft's Compound File Binary File Format (CFBF) or the older version OLE1.0 and OLE2.0 Formats, see also Wikipedia. According to the file command on Linux, version 2 is used. This format can be read via CompoundFileReader.
OpenMCDF's Structured Storage eXplorer allows a fast investigation of the file structure (Windows only).
Typically CFBF container store a custom CLSID in the root storage object to identify the applicable application e.g. OrCAD or Microsoft Office. However, OrCAD provides this CLSID in a generic application independent form as {00000000-0000-0000-0000-000000000000}. See 'Root storage object CLSIDs' for technical details. As a result it's not easy to identify an unknown CFBF container as an OrCAD design, e.g. via the Linux file application.