Sorry, I don't know the pass rate for the USMLE for DO schools. The board exam for DO schools is the COMLEX. So, that's what they would brag about. Some DO students DO take the USMLE but it is not a requirement for DO schools.
You could probably find out more on this topic at the Rotation forum.
According to the National Board of Medical Examiners in 1999:
FMG 53% (ECFMG schools)
Note: Osteopathic students are studying for (and focused on) the COMLEX during the testing period. All osteoapthic students MUST PASS the COMLEX which includes a section on osteopathic treatment techniques. This most likely explains the small difference between pass rates between the MD/DOs- many DOs typically take both tests but must focus on the format of the COMLEX rather than the USMLE I. Also note that the National Board of Medical Examiners has concluded that for licensing and educational standards purposes, the COMLEX=USMLE. So more frequently, allopathic residencies will accept the COMLEX in lieu of the USMLE. However, you must check on the individual program.