there's a brutal LOD system in place with FarCry - probably a bit too much for my liking (as in the first dropdown can be seen too readily).
Other techniques for such open spaces is shader degradation. Why resolve normalmap and specular contributions on pixels that are far away? Texture mipmapping... this also helps with pixel shaders that require per-texel lookups. Less work to do on shaders that are further away.
For the outdoor areas, it is not necessary to have a BSP (though I don't know what farcry does). Objects themselves could be used in occlusion queries for more granular occlusion of smaller objects in such large scenes (can't see the house? then the 20 child objects inside the house don't need to be queried for visibility). The terrain itself could be LOD'd based on quad/oct-tree approach...
but yeh, thats just a few approaches of the virtually unlimited ways a dev team may decide to implement occlusion and scene managment.
I've never looked into FarCry, so I don't know what (of many) technique/s they're using