I checked at familysearch.org (as you probably did) with no luck. I also checked the city directories. John O appears in 1870, but not in 1872. There no 1871 via ancestry. Unfortunately, back then they frequently did not include wives names, nor did they list death dates, as they did later on.
I would try either the Concord City Clerk, or the State Bureau of Vital records. You might try calling each to see what you can get from them. http://www.concordnh.gov/index.aspx?NID=994