Ōkubo Tadayo served as a retainer of the Matsudaira clan.  Tadayo was the eldest son of Ōkubo Tadakazu.  Tadakazu came from a branch of the Ōkubo clan located in Uewada in the Nukata District of Mikawa Province.  Tadayo and Tadakazu were among an elite group of retainers under Matsudaira Hirotada known as the Kanie-shichihonyari, or the Seven Spears of Kanie.  Tadayo was further included in a group recognized for their contributions to the formation of the Edo bakufu under Tokugawa Ieyasu, known as the Tokugawa jūroku shinshō, or Sixteen Divine Generals of the Tokugawa.

The Ōkubo clan came to serve the Matsudaira and Tokugawa clans through their relation to Matsudaira Kiyoyasu, the grandfather of Tokugawa Ieyasu.  Although Tadayo was from a branch of the Ōkubo, the status of Tadayo’s family surpassed that of his uncle, Ōkubo Tadatoshi.  In 1563, Tadayo contributed in battles against the Ikkō sect in Mikawa and, in 1573, the Battle of Mikatagahara as a member of combined Tokugawa and Oda forces against the Takeda.  After suffering a loss in the Battle of Mikatagahara, according to one account, Tadayo joined with Amano Yasukage to fire shots and stir confusion in the middle of the night among the Takeda forces camped at Saigagake to the north of Hamamatsu Castle.

In 1575, Tadayo and his younger brother, Ōkubo Tadasuke, along with Naruse Masakazu (a yoriki, or security officer), and Kusakabe Sadayoshi, received praise from Oda Nobunaga for sticking to the enemy at the Battle of Nagashino and received trumpet shells as an award.  Immediately after the Battle of Nagashino, Ieyasu made efforts to recapture castles in Tōtōmi Province that had fallen to the Takeda.  The capture of Kōmyō Castle isolated Futamata Castle.  Ieyasu assigned Tadayo as the commander to lead a force to lay siege to Futamata.  Tadayo established a base of operations at Tobayama Castle, and waited until the end of the year when Yoda Nobushige surrendered the castle.  Tadayo then became lord of Futamata and reconstructed the castle and a tower in anticipation of an attack by the Takeda.  In 1582, following the coup d’état against Oda Nobunaga known as the Honnō Temple Incident, Ieyasu expanded his influence in Kai and Shinano provinces.  Tadayo served as the sōbugyō for Shinano, defender of Komoro Castle, and guardian of Yoda Yasukuni, the eldest son of Nobushige.

In 1585, Tadayo participated together with Torii Mototada and Hiraiwa Chikayoshi in the First Battle of Ueda in Shinano, but lost to Sanada Masayuki and Yazawa Yoriyasu.  In addition to his military exploits, Tadayo was known to excel in matters of governance.  Tadayo persuaded Honda Masanobu to return after he rebelled against Ieyasu and was banished and rebuked Ii Naomasa for complaining about the lack of shōyu for potatoes served at the base during a battle.  In 1590, Ieyasu moved to the Kantō following the decimation of Gohōjō clan.  Meanwhile, Toyotomi Hideyoshi awarded Tadayo control of Odawara Castle and a fief of 45,000 koku.