From Halopedia, the Halo wiki
He was assigned to First Platoon, One-Alpha Squad, during the Battle of Harvest near Gladsheim. Shortly thereafter, Osmo was transferred to One-Charlie Squad because he yawned excessively and Staff Sergeant Johnson wanted to make a good first impression. He was the first militiaman to be killed in the battle when he was torn apart by an Unggoy named Yull.
However, he managed to shoot his MA5 assault rifle, alerting the other militia. This also caused one of Maccabeus' escorts to draw his Spiker and fire a spike into Rol Pedersen's chest, killing him. He was the first UNSC casualty of the Human-Covenant War.
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