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
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offline firefight coop surpports online 16 players, system link 16, and 4player split screen.
no you cant
i would imagine so considering that player character is apart of a squadron called Team Noble, however Bungie hasent officially released anything really about the campaign.
You can get 43 achievements offline on halo 3 im not sure on odst.
Halo Reach is definitely for single player. Campaign and Forge, you can do alone, but you will get the most amount of entertainment out of the multiplayer modes.