How do I get the the world matrix of the mesh based on the mesh space?

The vertex positions are given w.r.t. a reference space. This space is conventionally called the model space. Recomputing the vertex positions w.r.t. another space requires the existence of a model-to-other space transform. This transform must be explicit. It cannot be implicitly given by the vertexes themselves.

The only thing you can do is to estimate another origin by computing the center of all vertex positions (or something similar). This works for symmetric geometry reasonably well. Otherwise you need to handcraft the transform and add / apply it during import.

EDIT: L. Spiro's practical tipps ;D

Slam your head into a wall until you forget what OBJ files are