You are not the first person to complain about oxygen
I will say it's working exactly as designed, and if you're not able to fully explore planets in the beginning, that's good (IMHO). It works as a sense of tension and a motivator on a lot of levels.
It also undergoes what I refer to as a "phase change" over the course of the game. in the beginning, oxygen is super important. then you get better suits, and maybe it's your HP, or environmental damage that are your biggest concern. Then you discover deep caves, and oxygen's right back in there.
There are devices that effect oxygen, and plenty of stuff to balance it out. You're playing 7 sectors. The actual game is infinite!
So, I feel your pain, but think of the big picture: survive, press on, and planets will open up to you. you will forget all about that one planet in sector 2 you couldn't quite get, because you'll be knee deep in shipwrecks, temples, and caves, not to mention Dunwiigu Evangelists, Limoquee capital ships, and all the race quests...