From Halopedia, the Halo wiki
Omega Three is a UNSC emergency code to be used during combat operations. It is the final panic code, an order to break and run no matter what the cost. As such, Omega Three is used only in extreme situations, when a combat situation suddenly turns extremely grim for the UNSC forces and they would be massacred if they do not immediately retreat.
Omega Three was used during Operation TORPEDO, July 3rd, 2545 when seven Covenant cruisers arrived on Pegasi Delta to halt the Spartan-IIIs' attack on the Covenant refinery there. Except for Tom-B292 and Lucy-B091, who managed to jump into the water before they were hit, all of the Spartan-IIIs were killed by the plasma bombardment from the Covenant cruisers and any other survivors were finished by the massive explosion of the refinery.
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