It looks like the best and probably only option for OVS to B25 conversion would be to use Ovation Pro software from which both of these file types originate. Whenever this conversion is useful or even possible could not be verified.
Convert Ovation 1 style sheet file to Ovation Pro for RISC OS DDL file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert ovs to b25 on Windows
A desktop publishing program for Windows
Convert from ovs file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.