If you're a mapper, you should know that maps are built of solid brushes. When you compile that map into a
BSP file, a lot of data is removed. Faces that are in empty space outside of the world are removed, leaving only an inner shell where the player can explore. What's more, any place where two standard brushes touch a lot of face dividing occurs. I'm sure there's lots more, but those are the ones that come to mind right now. The bottom line is that the geometry that you can work with in
Hammer is very different to what is compiled into a
BSP file, and it's therefore incredibly difficult to reconstruct that geometry with a program such as a decompiler. When you compile, it's meant to be a one way trip, the game engine doesn't need all the data you or I need to create the map. You're better off trying to rebuild the brushes by hand.
If you want to investigate how that part of Opposing Force was done, all the point entities should still be there, and hopefully most (or all) of the brush entities, in one form or another. An alternative is to open the map up in a viewer like
Crafty, and look at the entities that way.
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