I recently found a great source of making O scale trees by using dead Salal bush branches found in the woods near my home. These bushes are a broad leaf evergreen that grow all over Whidbey Island and the Puget Sound in general in coastal areas. When the plant dies it reveals a very nice armature that is perfect for making tree models. The branches are varied enough that they could probably be used for nearly any scale depending on how you cut it. The photos below describe my technique for making a tree out of this material.