how oro killed gaara's dad.

[Minagatachi]

For all we know he could have have gained up on him with all the Sound Ninja, all the Akatsuki, all the Jinchuuriki and all the other Edo Tensei that Kabuto brought back in the war (Including Madara). Bottom line is that we don't know.

Orochimaru would have planned to kill him, and there is no way in hell that Orochimaru would have gone into the fight with a chance of him losing. Planning > Strength in this manga.

 

[Almighty Ra]

Befitting his status and responsibility as leader, the Fourth Kazekage cared deeply about the well-being of his village, so much so that he was willing to ally with Orochimaru to destroy Konohagakure to bring Sunagakure's resources back up, despite knowing Orochimaru is a highly dangerous criminal. This turned out to be a fatal mistake on the Kazekage's behalf, as he and his bodyguards were betrayed and assassinated by Orochimaru.

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It also states that A insinuated that the Fourth Kazekage's death may have been the result of a secret plot of Danzō Shimura with the Akatsuki, more exactly Orochimaru who at the time was no longer a member of the organization.