I had trouble getting Blender 2.79 to osgt exporting working fluently. Good people at OpenMW-discord instructed me what's this transparency all about, and so I incorporated alpha blending (blender's "Z Transparency") and alpha testing (blender's "Mask"), and backface-culling (Blender Game rendering the "backface-culling" checkbox) to osgexport. I'm still very much a beginner in graphics and 3d-modeling, so I'm not sure if it's perfect, but I'm getting results that satisfy. Note: This is nothing "official" OpenMW-stuff, just a way I found that works.
So, how to make a osgt-model with Blender to OpenMW?
1) Create a model in blender.
2) Add material or multiple materials to your model (there might be performance advantages in using the same texture file)
3) Add texture to your material, tune all settings of texture and material in blender until you are satisfied with the results. At least png works, probably some other file formats too.
4) Set Blender rendering to "Blender Game". If you don't want backface-culling for some material, uncheck material->game settings->backface culling.
5) Save. After saving, scale bigger to 1:128 (edit: or 1:70, hinted elsewhere). This is a workaround, as osgt-exporter doesn't yet understand Blender unit scale.
6) Don't save the scaled version, but export that to osgt.
7) Copy osgt-file to data/meshes, copy textures to data/textures. Check that your osgt-file has correct path for texture (should be "textures/filename.png"
8) Open OpenMW-CS and create a new plugin (or game), Objects -> Add Record, e.g. type Static (other if you want something else) -> At Model/Animation column, write your osgt-filename (it should suggest and auto-complete).
9) Open a Cell, drag&drop the Object to the Scene, that will create an Instance of your Object.
10) Save your game/plugin, open it through openmw-launcher, play and enjoy.
As a sidenote, some other learning experiences for me were:
a) Join all objects in blender to one before exporting, to reduce drawables, therefore getting insanely much higher performance.
b) Alpha testing should be faster than alpha blending, but textures might need tuning, you likely need a big enough resolution to get texture alpha-edges look nice. There's probably a ton more to learn.
c) I've had occasional rendering problems of alpha edges, at least with alpha blending, when ctrl-D duplicating branches and placing them over each other. These problems are visible in blender, so according to my experience, if you can make a good blender model, it works in OpenMW too.
d) Placing planes that cross each other sometimes resulted in rendering problems, I have no idea why, and I haven't tested this with the latest exporter settings. Because of this, I prefer to avoid planes that cross each other.