Source: Kaiho Kenjaku (懐宝剣尺) of 1797
Ō Wazamono (大業物) can literally be translated as "great instrument that plays as it should". It was the second highest of the following ratings:
Saijō Ō Wazamono (最上大業物), "Supreme Grade."
Ō Wazamono (大業物), "Excellent."
Yoki Wazamono (良業物), "Very good."
Wazamono (業物), "Good."
History of the ratings
The terms come from the book Kaiho Kenjaku (懐宝剣尺). The book was first compiled in 1797 at the request of Heisuke Takuseki, a famous sword appraiser and retainer of the Hizen Karatsu clan, together with Yamada Asaemon Yoshitoshi (山田浅右衛門吉睦), an expert test cutter and executioner of the Tokugawa Shogunate. He was the fifth head of the famous Yamada line of sword testers. A total of 163 sword makers achieved wazamono or higher status. Due to its popularity, the Kaiho Kenjaku was republished in 1805 and 1815.1
A major updated list was published in Kokon Kajibiko (古今鍛冶備考) or "Ancient and modern blacksmith notes" of 1830.2
Any test cut is very stressful on a blade, and thus there is a considerable risk of damage. Therefore many old masters were excluded from testing.
1. An original of the Kaiho Kenjaku (懐宝剣尺) can be viewed online in Waseda University Library's Kotenseki Sogo Database.
2. The Kokon Kajibiko (古今鍛冶備考) of 1830 can be viewed online in Waseda University Library's Kotenseki Sogo Database. (A simple viewer is here.)
List of Saijō Ō Wazamono (最上大業物)
The original 1797 list contained 12 makers and was as follows:
Osafune Hidemitsu (長船秀光) = Hidemitsu II
Mihara Masaie (四代三原正家) = Masaie IV
Osafune Motoshige (長船元重)
Nagasone Okisato (長曾弥興里) = Nagasone Kotetsu (虎徹) I
Nagasone Okimasa (長曾弥興正) = Kotetsu II
Seikan Kanemoto (清関兼元) = Kanemoto I
Magoroku Kanemoto (孫六兼元) = Kanemoto II
Tatara Nagayuki (多々良長幸)
Miyoshi Nagamichi (初代三善長道) = Nagamichi I
Sendai Kunikane I (初代仙台国包)
Hizen Tadayoshi I (初代忠吉)
Hizen Tadayoshi III = Mutsu no Kami Tadayoshi (陸奥守忠吉)
The major 1830 revision saw Seikan Kanemoto demoted to Ō Wazamono, and two new smiths were added:
Osafune Kanemitsu (長船兼光)
Izumi no kami Kanesada (和泉守兼定)
List based on the Wikipedia List of Wazamono which in turn was based on the interpretations of the list by Kōkan Nagayama; The Connoisseur's Book of Japanese Swords. Kodansha International. 1997 p. 37. And Markus Sesko; Swordsmiths of Japan. Lulu Publishing 2014. Notice that it is not always clear which smith is mentioned and that many smiths also worked under aliases. The last 67 added smiths are from jp-sword.com, which are in turn based on the original editions.
A complete list of wazamono can be found in the main glossary article Wazamono (業物)