Correct myself. Apparently OPTech v0.6 (just released) can import OBJs which have texture co-ordinates so now I'm wrong about the possible ways of texturing an OPT
OPT files are made by combining a DXF (model) with 256 colour BMPs (textures) and adding other information such as weapons hardpoints in either ACE or OPTech.
Further information on the ship is held in the executable (speed, hull rating, shield rating, warhead capacity..etc) which can be edited with either BinHexEdit or MXvTED.
Further information, some OPT files, and some links at http://www.btinternet.com/~jmdare/
(which is my site)