I think we'd all like it to be an OVM buckle, but the odds are that it's not. Here's why. Ohio raised 26 regiments of infantry early in the war for 3 months' service. 13 became US volunteers and converted to 3 year troops in May-ish (1st - 13th Regiments Ohio Volunteer Militia). Ohio retained nine regiments more at its own expense for 3 months' duty in western Virginia and Kentucky, and kept four more as a state reserve force - again for 90 days. The state provided their uniforms and equipment. By mid-summer 1861, the 3 month state units were demobilized and converted into 3 year federal units which, by that time, the USG was able to reassert itself in raising and equipping an army. As a result, once a regiment mustered into service as a federal organization, the USG provided all necessary arms and equipment, not the state.
When the time the 91st mustered into federal service a year after the war started, Ohio had met its recruiting responsibilities and (officially/generally) and had little else to do with the regiment. As federal troops, the unit complied with Army regulations to wear/carry what was issued to them; to include US plates and buckles.
It's possible (although unlikely) that Compliment somehow acquired an OVM buckle for his image, but that's a long shot. BTW, don't confuse the federal OVI, (Ohio Volunteer Infantry), with the state OVM, (Ohio Volunteer Militia); they're two distinct organizations.
Do you have a copy of Darl L. Stevenson's book, Headquarters in the Brush, Blazer's Independent Union Scouts (Athens, OH: Ohio University Press, 2001)? Blazer was with the 91st. It's the only easily available history of the regiment. In 1865, A.H. Windsor wrote a short pamphlet cleverly titled, History of the Ninety-First Regiment O.V.I (Cincinnati: Steam Printing House, 1865), but it will likely be a challenge finding a copy. Aside from some poetry written about the 91st's exploits, I am unaware of any other published information on the unit.
I hope this helps a bit.
jtb