The OPJ file is the project file associated to a specific design (.DSN). Generally, the file names of the OPJ and DSN files are the same. The OPJ file is an ASCII file, that should not generally be manually edited, as it could harm the design file.
The OPJ file contains settings, scripting and links to external files that are relevant to the specific design file. The OPJ file is also used by simulation applications, such as PSpice. PSpice adds links to libraries, settings and the simulation to the OPJ file that support PSpice simulations.
Capture stores settings and file links for DRCs, Netlists, BOMs, the design filename, etc. in the OPJ file. Settings saved in the OPJ file include items such as the “Header” line and the “Combined property string” in the ‘Line Item Definition’ section of the Bill of Materials window.
Although the OPJ file is used by Capture designs and libraries, it is not critical to the design, as it can be recreated and the files relinked without issue to the design or user.
The OPJ file is especially important for PSpice. As stated earlier, some of the linked files support the simulation files for the design. Although, it can be rebuilt, it is more difficult to recreate the settings that support a simulation.