I know that there were at least two DO's in the med-psych program at University of Iowa. I think that someone coming out of a DO program would be just as competitive at a combined residency program as they would be for either department's residency program separately - if that makes sense.

I really don't know if those DO's had taken the USMLE in addition to the COMLEX - that's just out of my area of knowledge. Perhaps there is a DO out there who can help out?

I am not aware of any osteopathic med/psych programs, although the military med/psych programs at Tripler (HI) and NCC (MD) would likely "count" as osteopathic GME under the special agreement between the military and the AOA. You would just have to submit some paperwork to the AOA, according to some DOs I know in the military.

You may know this already, (I can't tell if you are a medical student or not from your posting), but many DO students take both the COMLEX and the USMLE if they are planning to apply to allopathic residencies. I do know that some allopathic programs are said to be willing to consider COMLEX scores alone, while many others simply will not. This is a great reason to research the individual programs specifically in advance!