Original Sins requires everything from the games assets to be inside your mod folder to function correctly (Check out other Original Sins mods like SoA 2 to see examples). That is the only pain in the ass about it, because it makes it harder to keep track of what you have changed, or if something goes wrong it makes it harder to track down the problems.
Like Lavo said. You are much better off modding Entrenchment, Diplomacy, or Rebellion, because they only require the modded, or changed files in your mod folder (plus the manifests).
If you really want to mod original sins then copy these folders from the original game, and put it in your mod folder. The GameInfo, Font, Galaxy, Mesh, Movie, Particle, PipelineEffect, Sound, String, TextureAnimations, Textures, and Window. For the steam version of trinity you may need the AppData folder copied too.
I know it seems ridiculous to copy all of that over, but if you dont you will minidump every time.
Then you can use the reference files to replace the binary files in game. Just keep track of what you change, because if you do get errors you are going to need to know exactly which file you changed, and what you changed in it.
