Unfortunately the Compiled Service Records for Ohio units have as yet been published by Footnote.com.

Your best bet would be to order his records through the service noted in the Red enclosed box above.

There has to be some story in his short career, promoted to 1st, Orderly Sergeant, and soon thereafter reduced to the ranks.
The First Sergeant was the highest ranking enlisted man in any one company and was the equivalent of the Enlisted executive
officer in the company.

Here's all I could come up with:

James Callahan

Residence was not listed; 23 years old.
Enlisted on 8/29/1864 as a Private.
On 10/7/1864 he mustered into "G" Co. OH 181st Infantry
He was Mustered Out on 7/14/1865 at Salisbury, NC
Promotions:
* 1st Sergt 10/7/1864
* Private 12/7/1864 (Reduced to ranks)

Sources used by Historical Data Systems, Inc.:
- Official Roster of the Soldiers of the State of Ohio