A freeholder NSO was a commonly used term from the 1830s to 1890s. It segregated different types of freeholders.
A freeholder NSO meant a person who owns property for the purpose of "boarding out" (we know it as renting out), in London people listed as "freeholder NSO" were called lodging house keepers, who ran homes that they "boarded out" (rented) rooms to people in order to earn an income. Sometimes, this is commonly found in London, lodging house keepers ran homes that acted like brothels.