Assuming you could make a sizable contribution, extra gas would eventually dissipate into space. That said, oxygen is high in potential chemical energy (usually contributes energy to a reaction, hence everything oxidizes), while CO2 is low (doesn’t react with much without external energy like sunlight). This the process of making oxygen or using (storing) CO2 are roughly equivalent energy wise. Of course processes and catalyst may change the exact numbers, but you aren’t getting around the basic thermo problem.