The OSGB output will depend on the quality of your texture mesh in the first place. If you wish to obtain results with the highest resolution, you can use the Full processing template: Full Processing vs Rapid / Low Resolution.
Subsequently, you need to tweak the parameters Number of Levels and Texture Quality.
The Number of Levels allows defining the number of different levels of details to be generated and it varies between 1 and 7. The higher the number of levels, the more detailed the representation and the longer the processing time. Please note that, for large projects, it might happen that the LOD cannot be generated for a high number of levels as there is a maximum number of 20000 triangles that can be generated for each level of detail.
The Texture Quality allows defining the resolution of the texture. It is possible to select Low (512x512), Medium (1024x1024) and High (4096x4096).
Hopefully this helps.