This was honestly never a problem for me. There's so many electrolyzer recipes that produce oxygen that I'm surprised its a big issue for you.
At the end of the day I can't really see a strong argument to add more oxygen sources: if you check NEI recipes for oxygen you'll find there's already a ton of them, and you're really going to need to run some of those processes in order to advance anyway. For instance: if you aren't making stainless steel yet, you may as well start your chrome production now. You'll need a ton of it, and one of the primary sources is ruby dust. Which also produces oxygen.
A quick google suggests that Obsidian is a decent source of oxygen. Not confirmed.
yeah thanks Obsidian is a good solution I didnt see. And I honestly dont know why I have too less Oxygen. Im using tons of steel for all the Railcraft stuff, maybe thats why I dont keep up.