The ideas posed in the first part of this post would be the point where I'd leave things for an Aussie Freeform. A few archetypal ways of manipulating the situation, a few goals that point certain characters in certain directions...then you just let the players go. A GM per 5-10 players (plus a head GM), and if decisions need to be made, the outcome is determined by whatever is best in the context of the overall story, even if things look like they should head a different way at the immediate scale. Maybe throw in some egg timers for specific effects that require delicate work (like hacking the server, tending to someone's wounds, fixing the power, or adjusting the oxygen-beds). Players can't do anything else while the sands in the timer are running through; and if they are disturbed, the count resets. Combat would be enacted through basic wrestling, each player has a literal bottle on them representing their oxygen supply (maybe attached by loop velcro to a hook velcro belt), if one player can remove the other player's oxygen, they've won the conflict. Stealthy players might just rip oxygen bottles from other players when they walk past... Maybe to reflect things with a chance of success or failure, you have one of those games where you hold a wire loop and need to pass it along a twisting wire without touching the wire, buzzers and lights go off if you fail the task and you need to start again.
For something a bit meatier, a bit more abstract and a bit more accessible to less dexterous or physical players, I'd consider adding a simple test mechanism. Rock-Paper-Scissors is always easy because you don't need to carry a deck of cards, and you don't need a flat space to roll a die on. This is all getting away from the Australian freeform angle of the game, but might make it more approachable to other types of players. Maybe a basic pair of attributes... mind and body, on a scale from 1 to 4. When two players have opposing goals (or when one player confronts a task with a specific difficulty value) they declare their intentions, then compare their numbers.
If the two opposing sides have equal scores, a simple RPS test occurs. If either side wins, their intentions are enacted first. If the opponent's intentions are still valid, theirs may occur next...but generally if the two characters have goals that could occur simultaneously, there's no real need to test things.
If one side has a score 1 point higher than the other, their intention is enacted on a win or a tie. The side with the lower score only succeeds if they specifically win the conflict.
If one side has a score 2 or more points higher than the other, they automatically succeed.
In most cases, characters will have scores of 2 and 3, it's only in a few rare circumstances that 1s or 4s might be in play for a character on their own. But if two characters work together to a common goal, they might use the highest character's score +1 per person helping them. Characters might have special equipment that gives them a +1 in specific circumstances...Similarly, a character might waste a bottle of oxygen for extra exertion in a physical action, gaining a +1 in the process.
Injured characters would lose temporary points from their physical statistic, characters losing their sanity would see a reduction in their mental statistic, characters suffering oxygen deprivation (on their last bottle) would lose a point from each.
I wouldn't do anything more than this with regards to game mechanics. That's complicated enough... this game is more about the interpersonal drama and conflict than anything else.
- working together, but there's a risk that skill attempts won't work.
- sacrificing the members of the crew who are deemed "expendable".
- one person stockpiling oxygen for themself
- possibly others I haven't considered (I'd give bonuses for this)
This isn't an "Alien" situation where characters are picked off one by one, via some horrific monster. It's a slow burn psychodrama in a potentially zero sum environment, where things will go horribly wrong for those who choose to do nothing.