Okay, I tried it again, and again the same sh.. happened:
The oxygen went down, despite the fac that my ship wasn`t damaged and the oxygen generator was fully functional!
I died for no Motherf..... reason!!!
It must be a bug or the game hates me!!
It is not a bug, there are a few situations in the game that alters how much power you have available, the most obvious one being while fighting in certain nebulas that cuts down available power (the bars become blue and unavailable). You must pay attention to everything all the time. Pause the game at least once every minute to look at how your crew is doing, which doors are open, how your power is distributed, damaged systems and hull breaches. Every time you jump to a new place, read all texts carefully and make sure you understand what they are saying (for example, if a nebula is slashing your available power in half). Pause the game, assess the situation, adjust as necessary.
This game has a degree of randomness to it but if you feel you are at the mercy of it's whims then you are not giving it your best. You need to have an adaptable plan for how to deal with all the possible situations you may encounter. With experience you start to learn what kind of challanges you can typically expect from different kind of star systems and levels, and you have a loose plan for how to progress through the game, taking advantage of and changing the plan at a moments notice to make use of whatever the game gives you. It's a lot like playing a game of cards. The draw is random but you know which combinations are good to have so you try and leave a few good options open in case the right cards eventually get played on to the table. That's the best analogy I can think of actually, the abstract challange of FTL is a lot like building the best poker hand with what you have and using it to win.
If you think the game is throwing impossible encounters at you in FTL, it means you have not done all that you can to maximise your loot, scraps and load out. It means you progressed too far into the game without addressing a weakness in your ship design or crew selection. Same thing with Fallout, if you get killed in random encounters it could mean you have not geared up and developed your characters well enough to handle the area you are in, and perhaps you are handling the encounter all wrong. Yes, sometimes FTL and Fallout generate unproportionally difficult challanges, I'm not saying you can win every battle. I am saying that these are exceptional circumstances, they do not represent the true spirit of these games. If you know the dangers ahead (from experience), you should have a whole ABC of plans and a rough outline on how you intend to progress through the games.