go to Custom games then go to network it will say:Xbox Live
System Link
Local
Select local
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You can get 43 achievements offline on halo 3 im not sure on odst.
no you cant
yes but it is really hard you have to go onto online then choose how many players you want that is how you do it
Unfortunatly you can't.
offline firefight coop surpports online 16 players, system link 16, and 4player split screen.