I have done both. As I recall, I was the one who figured out how to globally add objects back in the early days of OpenZone. We have several custom objects on our server including custom aug pools, custom PoK "book" (its a globe), and assorted other objects including our stargate.
It's much easier now. To add objects, you export an s3d file of just those objects. Add that file to the GlobalLoad.txt file.
As for mob models, there are several that come with OZ now, so you can use them as examples and either make an s3d with it or include it into the zone you are going to use it in. Again, to make it global add the file to GlobalLoad.txt.