Levels are not the same as they used to be. Ogre is a scene based engine. This means that all levels are modified in modelling applications and assembled using the planned in-game editor. Here is an example workflow using QERadiant or another "BSP" modelling tool.
1. Create the shell of the level using QERadiant. This consists of all the rooms and textures.
2. Create all static meshes and actors in Milkshape or another supported modelling tool
3. Open RF2
4. Press F10 to get into editor (this is not working yet)
5. Start a new scene
6. Place your static meshes, lights, particle systems, and actors around the level using the in-game editor.
7. Save your scene. It will save into an XML based format.
Static meshes are meshes that are not animated. So trees and buildings are static meshes, yes. If you animate the tree, then no.
If you need walls, you should consider creating your level using Blender or another modelling application, then exporting to Ogre .mesh or .scene format. This is the most recommended method. Reason being that there are no more "leaks" or any other anomolies using this method.
BSPs can be used and are made using the Quake 3 map editors. These are open source now so there are no license restrictions to content created with them anymore.
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