Oxygen and improved blast doors

[SpikeRosered]

Improved blast doors save you money, period. You will be boarded during the course of the FTL game even if you run from every encounter as you are bound to eventually hit one of the "You've been boarded!" beacons. Every time those boarders destroy a system you take hull damage which you have to pay for. The more efficiently you can kill boarders the less damage you take from them, thus saving you money.

Furthermore unless you have a surplus of crew you'll likely have to pull crew off of systems in order to save them from boarders which makes your systems inefficient and thus you are more likely to take damage during ship battles which costs your money.

Maybe I'm a door freak, but I almost always use my starting 30 scrap to buy the first upgrade for improved blast doors. Fires and boarders are two of the most annoying things to deal with, and better doors slows down both.

[The Captain]

Getting L2 doors is important, but not as important as your 2nd layer of shields or L4 engines, IMO. If in a Mantis sector I might make an exception an upgrade doors before getting to L4 engines, but not necessarily. Nearly always I'm first upgrading to 3rd bar of shields, then 4th, then 2 power. Not getting hit by weapons is far more important than the occasional damage I might take from fire or boarders.

[LeadHead]

Oxygen - I upgrade it only when I have all my essentials at my wanted levels or my power generator is maxed. If you get shot in the oxygen you can keep it damaged for a bit without sending your crew to that room.

Doors - Never beyond lvl2.

[UltraMantis]

I'd agree that lvl2 Shields is generally the smartest thing to do. It prevents damage and that means less scrap gone to repairs.

Now Oxygen, what i like to do is shuffle power a lot. With a Life Support upgrade replenishing the ship's oxygen supply is a breeze (ooh, a pun!) and i like to abuse that by switching O2 off during combat, and back on after the fight is done. This allows me to borrow 1 power bar and use it elsewhere while fighting. The risk associated with this is taking damage while oxygen levels are low, but at least the system is a bit more damage resistant.
I call it 'free power', the crew calls it lunacy.

Doors i rarely upgrade because the lvl2 upgrade is almost worthless as it won't stop a determined assault while lvl3 will fare much better. Unfortunatly having such upgrades early on is unlikely on most ships. I'd rather upgrade Shields or Engines or something ship-specific (Teleporter, Cloak) than spend on doors until later stages of the game. While it's not the best idea, my practise is to buy lvl3 doors only before the Flagship fight.

[obliviondoll]

I'd agree that lvl2 Shields is generally the smartest thing to do. It prevents damage and that means less scrap gone to repairs.

Now Oxygen, what i like to do is shuffle power a lot. With a Life Support upgrade replenishing the ship's oxygen supply is a breeze (ooh, a pun!) and i like to abuse that by switching O2 off during combat, and back on after the fight is done. This allows me to borrow 1 power bar and use it elsewhere while fighting. The risk associated with this is taking damage while oxygen levels are low, but at least the system is a bit more damage resistant.
I call it 'free power', the crew calls it lunacy.

Doors i rarely upgrade because the lvl2 upgrade is almost worthless as it won't stop a determined assault while lvl3 will fare much better. Unfortunatly having such upgrades early on is unlikely on most ships. I'd rather upgrade Shields or Engines or something ship-specific (Teleporter, Cloak) than spend on doors until later stages of the game. While it's not the best idea, my practise is to buy lvl3 doors only before the Flagship fight.

I tend to alternate between having full power to engines while taking fire, and a point or two in O2 between incoming shots so I don't kill my crew off without needing boarders to do it for me.

[Twinge]

Doors i rarely upgrade because the lvl2 upgrade is almost worthless as it won't stop a determined assault.

Wat. You're a madman Doors 2 are a very strong impediment to boarders, slowing their movement substantially and generally letting you injure to kill them without relocating any crew. And almost as important, they practically stop the spread of fire between rooms. All this for a mere 20 scrap. I don't generally rush it, but it's a solid upgrade that does a ton of work for the low cost.

[SmoothPapaJ]

Only thing I'll add as someone who never used to upgrade O2: sometimes after a catastrophic battle and O2 failure, when you've just barely repaired the O2 system but the ship's still out of air for a few seconds, the only thing that will save your crew is an immediate upgrade to the O2 for faster refill.

[UltraMantis]

I am a madman, that's why i have no winstreak

I like to mentally relax and belive the doors will hold. That requires lvl3 as it is so much better at it's job. lvl2 doors have failed me plenty of times. 2 skilled Mantis will rip them apart, and a Boarding Drone will not be denied long.
For 20 scrap they will stop fires from spreading and will hold unskilled non-Mantis boarders for a while. That is more than good enough i have to agree. It's just that early on those 20 scrap are badly needed elsewhere, and i am confident enough that i can deal with boarders in other ways.

Depending on ship room layout and crew size, i may well reconsider but for the most part i happily ignore the doors. Engi B cannot vent Piloting without venting the MedBay for example and is generally tricky to defend, so i may end up with lvl2 Doors sooner, but will also aim to quickly get lvl3 and be as safe as possible.

It a personal preference, not an objective guide/rule. In my case it's all or nothing when it comes to Doors so i view lvl2 as almost worthless.

[BirdofPrey]

For redundancy purposes, I like to have everything to level 2. O2 is a low priority repair since I don't need O2 if my shield go offline and i lose the ship etc. and an extra bar lets me hold off fixing it longer since it's still working after taking damage. It also lets you refill quickly after venting or having it off during a battle which can be good to have if you need O2 replenished in combat (out of combat you just walk away for a couple minutes). Level 3 is usually something of a luxury purchase.

As for doors, again for redundancy L2 doors are great since even L1 doors you get with a half damaged system is better than no doors, plus it helps prevent fires from spreading. I do often go to L3 doors though. It depends on crew. if I have some extra crew that can put fight boarders then I will use L2 doors for the most part unless I have spare scrap, but if I have a crew of zoltan and engis and/or can barely keep my systems manned then I get L3 doors so I can space boarders (sure you can do that with L2 but it's a lot harder since they break the doors much more quickly). Also, if you are relying on venting to control fires and boarders, a better O2 system is good to have.

I also upgrade sensors since being able to see what power is in what systems can be useful for targeting, and I always try and get l2 cockpit relatively soon. Even if I (almost) never use the unmanned bonus it makes me less vulnerable to having that system shot out and eating 0% dodge which is liable to get you killed.

[Gecimen]

L2 of any system gives it double durability so oxygen l2 is a must for a smooth game.
Blast doors are good if you fight in that solar flare scene.