I searched through David N. McBride's "Marriage Records of Highland County, Ohio (1805-1880) and didn't find a record of the marriage. The only thing close is a marriage of Absolom Roush to Sarah Walker July 10, 1831 by John Shafer, J.P. There are several marriages to Donohoo brides, but no Nancys and none to a Roush.

I did not see a will for John Donohoo in McBride's "Wills, Administrations, Guardianships and Adoptions of Highland County, Ohio (1805-1880)" but "C. P. Donohoo" appears as a witness to the will of John Roush, Sr. of Salem Twp. Date: Feb. 26, 1848. Not sure if that's of any use or not.

There is a rather extensive genealogy of the Roush family published in Elsie Johnson Ayres' Pioneer Sketches and Family Genealogies book. I can scan the pages for you if you want. At least two Absalom Roushs are mentioned as children but no marriages for them are given.