Let me explain:
The Oxx textures are a combination of 2 textures. The RGB channel is an ambient occlusion map (prerendered details/shades/scratches/rust), and is mapped onto the model using a secondary UV-set (from TMAP). It gives large surfaces more diversity (which would look very plain with only 1 texture, see ship hulls in SH3) and the prerendered shadows can emphasize dark areas.
The alpha channel is a height map, and is used for bumpmapping (as are Nxx textures) which is a way to 'fake' depth on a flat surface (for instance door handles, window frames, rivets).
Here's a screenshot of the Txx texture and Oxx texture (RGB-channel!) mapped onto the Yamato:
http://www.subsim.com/radioroom/show...7&postcount=26