What is the biggest supplier of oxygen on earth?

Could you elaborate on what entity or process is responsible for being the largest supplier of oxygen on our planet? Given the vast diversity of ecosystems and biological processes, it's fascinating to consider which one dominates in this vital aspect of Earth's atmosphere. I'm particularly interested in understanding the mechanisms behind this process and how it sustains our biosphere. Could you break down the science behind this, perhaps highlighting any specific organisms or systems that play a pivotal role?