Ōkubo Tadataka served as a bushō from the Sengoku to early Edo periods.  He was a hatamoto, or direct retainer, of the Edo bakufu.

In 1560, Tadataka was born as the eighth son of Ōkubo Tadakazu, a retainer of the Tokugawa clan, in Kamiwada in Mikawa Province.  His childhood name was Heisuke.

Tadataka served Tokugawa Ieyasu, the sengoku daimyō of Mikawa.  In 1576, Tadataka joined his older brother, Ōkubo Tadayo, in the campaign to pacify Tōtōmi Province.  His first experience in battle occurred at Inui Castle.  Thereafter, he fought in numerous battles under the command of his older brother.  In 1585, at the First Siege of Ueda Castle, the army served under the direction of Sanada Masayuki.  His older brother, Ōkubo Tadayo, acting upon orders of Tokugawa Ieyasu, served as a guard for Yoda Yasukuni who was participating in his first battle and, at the age of thirteen, had inherited the headship of the Yoda clan in Shinano Province.  In the eleventh month of 1585, after Ishikawa Kazumasa absconded, Tadataka (who was serving in lieu of Tadayo at Hamamatsu Castle) entered the base of Yasukuni at Komoro Castle.

In 1590, after the Conquest of Odawara, his lord, Tokugawa Ieyasu, moved to Edo while his brother, Tadayo and nephew, Ōkubo Tadachika (Tadayo’s son), were appointed as the lords of Odawara Castle in Sagami Province with a fief of 3,000 koku.   In 1600, at the Battle of Sekigahara, Tadataka served under Tokugawa Hidetada and participated in the Second Battle of Uedahara.

After the war, his second oldest brother, Ōkubo Tadasuke, was appointed as the lord of Numazu Castle in Suruga Province with a fief of 20,000 koku, but Tadasuke’s eldest son and heir, Ōkubo Tadakane, died early so Tadasuke adopted his younger brother, Tadataka, to become his successor.  Tadataka firmly denied the appointment on the basis that he did not merit the succession so, after the death of Tadasuke, the Numazu domain was abolished for lack of an heir.  Thereafter, in the course of political conflicts in the Edo bakufu, Tadachika was outmaneuvered and removed from his position.  Tadataka was also removed on account of complicity in the events.  He was then engaged by Ieyasu to serve as a hatamoto, or direct retainer, of the Tokugawa and received a fief of 1,000 koku in Nukata in Mikawa Province.

In 1614, during the Winter Campaign of the Siege of Ōsaka, Tadataka served as a magistrate for the corps of spear-wielding soldiers.  After the death of Ieyasu, he followed Tokugawa Hidetada, the second shōgun, to Kyōto and, in the era of Tokugawa Iemitsu, the third shōgun, served as a magistrate in charge of banners.  Around this time, his fief was increased by 1,000 koku.  From around 1635, he moved to the Kashima District of Hitachi Province with a fief of 300 koku and focused on drafting the chronicles of Mikawa.

Tadataka died in 1639 at the age of eighty.  Just prior to his death, Iemitsu inquired whether to increase his fief by 5,000 koku, but he declined the offer citing that, at his advanced age, it was unnecessary.  His grave is at the Chōfuku Temple in the town of Ryūsenji in the city of Okazaki in Aichi Prefecture.  He also has graves at the Honzen Temple in Kyōto and the Ryūgyō Temple in Tōkyō – commonly referred to as the Ōkubo temples because he built them.

Anecdotes

Tadataka is well-known for having deep knowledge and opinions on many subjects.  Nevertheless, in an era when riding in a palanquin was forbidden by individuals below the rank of hatamato, there is an anecdote of Tadataka being carried up to a castle in a large basin.  There are also anecdotes of Tadataka being frequently scolded by Tokugawa Iemitsu, the third shōgun of the Edo bakufu.  These stories are deemed to have been created in later eras.  It is surmised that these depictions of Tadataka as a heroic character were engendered by bushi dissatisfied with the tranquil order imposed by the Edo bakufu at the time when the Mikawa chronicles were written.

Without reflecting on his own ascendancy, Tadataka regularly helped numerous rōnin, or wandering samurai, to find new positions and was adored by many people for his chivalrous spirit​.

The well-known story of Ōkubo Hikozaemon (Tadataka) and Isshin Tasuke (a fictitious character) originated from a dramatization in a kabuki play written by Kawatake Mokuami, a disciple of Tsuruya Nanboku, a kabuki actor and producer from the late Edo period.

It is highly likely that the story that the attack on Tobigasuyama fortress in Mikawa during the Battle of Nagashino was an embellishment and not the first experience in battle for Tadataka.