Ooskoo is one of the main protagonists of Rise of the Spartans, he is the brother of Rahgath, the main villain of the series.
He taught his brother Rahgath how to fight with the energy sword, he, Ragath, other Rotam brothers, and other elites fought together against the UNSC and won many battles.
oskoo never believed Ragath was worthy, but he stated in ROTS part 9 he did this to only keep Ragath safe. The other Rotam brothers mistreated Ragath, and also did not believe he was worthy. Ragath was only treated fairly by Ooskoo. The other Rotam brothers believed Ragath was too young and not ready to become a worthy sangheli warrior.
Ooskoo is betrayed by Rahgath when Ooskoo says Rahgath isn't worthy.
Ooskoo, along with some of his crew, survived Rahgath's betrayal and assist the humans in part 9 in stopping Rahgath's plan.