- Lord Aldemar
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These ODSTs are not engaging in extra-atmospherical operations. They are primarily ground infantry. A possible explanation for the previous ODSTs in Halo 2, and Halo 3 is that they didn't think the atmosphere was breathable on The Ark and Delta Halo. Hence the more sealed suits.
I do not recall the ODSTs actually participating in the Fight for Reach. However, I think you are remembering when John and his squad infiltrated the Rebel base in the asteroid, equipped with suits similar to the ones ODSTs wear. These had 15 minutes of oxygen.
Mark V MJOLNIR had 90 minutes of oxygen.
Again, to recap. Halo 3: ODST has the ODST in non-vaccuum suits because of aesthetic choices; and just operational parameter choices.