Oxygen is emitted by plant life all over the ocean, but it can only be collected when there is sufficient pressure, generally at a depth of 150 or more.
To collect oxygen, simply gather bubbles, as each one contains minute amounts of the gas.
refueling 7000 of your oxygen per 1 item
Can be found at:Edit
Aquatic Planet: 150-250 depth
NB: You get this gas randomly from bubbles, so just take the bubbles everywhere. Plus don't use oxygen to refill your tank just collect bubbles to raise your oxygen