Tachi O'Hara served as a second lieutenant in the defense force of the Autonomous Republic of Munzo, where he developed romantic feelings for Club Eden singer Crowley Hamon. Exploiting his docking bay station, he aided Hamon in facilitating Casval Rem Deikun’s escape from Munzo.

By the One Year War, Tachi attained Lieutenant Junior Grade in the Principality of Zeon’s Intelligence Agency. He risked personal safety to evade Kycilia Agency surveillance and deliver critical intelligence to Sayla Mass, affirming his covert engagements. Driven by loyalty to Hamon, he joined her mission to avenge Ramba Ral’s death near Cuzco, piloting a Zaku I mobile suit against the Federation’s White Base.

In the assault, Hamon commanded Magella Attack tanks while Tachi operated Ral’s sole surviving Zaku. Their strategy involved ramming an explosive-laden Gallop-class land battleship into White Base. When Amuro Ray’s Gundam countered, Hamon recalled Tachi from boarding White Base to repel the Gundam’s advance on the Gallop. During combat, Amuro maneuvered Tachi’s Zaku into the line of fire from Hamon’s Magella Top, destroying it. Alternate accounts state Job John’s Guncannon immobilized the Zaku I before Danny’s Guntank obliterated it with autocannon fire.

Tearfully, his final moments expressed enduring devotion and hope that his actions served Hamon.