Tips for finding and creating oxygen in the early game.
The area where you spawn with all of the drop pods usually has a couple Air Tanks on the ground and Scrap Heaps lying around. These Air Tanks will replenish about 1/3 or more of your air, so look for these and pick them up.
There are also more Scrap Heaps throughout the world shown on the map as a white octagon, always loot these. They contain food, medi, thermo, or oxygen. Save these tanks for when you are desperate and need a big boost to carry you to the next spot.
Look for Gas Blossom vines, and the Air Sacks that grow off of them. The Gas Blossoms will also drop an Air Sack if you kill them. These Sacks stack in your inventory to use for later but they are organic so they do decompose if you wait too long. Always pick these up when you see them.
Urchins will drop Urchin Lungs which you can take air from so pick those up as well when they drop.
There are oxygen based gear that will help with consumption. Craft these if you are still having trouble with depletion times.
Start your base with the Workbench, add bio generator, refiner, then Oxygenator. Keep the generator full and you will always have a place to go for oxygen replenishment. This is usually the first thing I do when I play, total game changer.