There is a lot of information available on OBDII, but to some extent, what's available depends on the model. If you're referring to OBDI, that was much more manufacturer-specific as to what information was available and the format of that information.
OBDII standardized a lot of the "generic" information yet still provided opportunities for the manufacturer to have their own specific data as well. While the format is rather universal, there are still different protocols used throughout the years and models.
-Rod