Hi Seyeroteb! Thanks for giving us a shot and the feedback!
For oxygen, you are able to loot batteries that restore power to your dive-suit. Two of these can be found in the crate at the first objective and then periodically in other points of interest.
For defense, after completing the first objective and gaining access to the main habitat, you can find a shock-prod laying on the save station. Additionally, there is a harpoon gun on the map that offers you further range for attacks.