No, submarines do not have unlimited oxygen. Submarines are equipped with a limited supply of oxygen, and it needs to be replenished periodically to sustain the crew onboard. The oxygen supply on a submarine is typically stored in compressed gas cylinders or generated through onboard systems, such as oxygen generators or electrolysis.
The amount of oxygen carried on a submarine depends on its size, purpose, and mission duration. Submarines are designed with systems to recycle and conserve oxygen to maximize its usage. This includes removing carbon dioxide and other contaminants from the air and reusing the oxygen within the closed environment of the submarine.
However, even with these measures, the oxygen supply onboard a submarine is finite and needs to be replenished during extended missions or when resurfacing for crew rotation and resupply. Submarines need to resurface or connect to a surface support vessel to take on fresh supplies of oxygen and other essential resources.