Oxygen is a common chemical element and can be found in many things, the most obvious being water buckets.
Oxygen is a nturally occuring element in Minecraft. Many items or products that contain components derived from plants will produce cellulose, which will produce five vials of oxygen. Tallgrass, excluding its variants, will produce shikimic acid, will also produce five vials of oxygen. Oxygen can also be produced as a potnetial by-product form the decomposition of blocks including stone. Blocks such as sand and quartz - which are both variants of the same chemical, can be decomposed into silicon dioxide, which can be further decomposed into elemental silicon and two vials of oxygen.