MIT News reports an MIT-designed device, known as the Mars Oxygen In-Situ Resource Utilization Experiment, or MOXIE, has been generating oxygen on Mars since April, 2021, about two months after it landed there with the Perseverance rover.
The device extracts carbon dioxide from the thin Martian atmosphere through a HEPA filter, then compresses it and heats it up to about 800 degrees, then sends it over "a nickel-based catalyzed cathode" made by a company in Utah to generate oxgyen ions and carbon monoxide - generating about as much oxygen as a "modest tree" on earth would, or about six grams an hour.
The ultimate goal is not to terraform Mars, but to develop a system that could generate enough oxygen to sustain any human explorers - and provide the fuel they'd need to get back to earth.
Researchers envision that a scaled-up version of MOXIE could be sent to Mars ahead of a human mission, to continuously produce oxygen at the rate of several hundred trees.
The device sent to Mars with the rover was designed to be as small as possible, more to test the basic principles.
The current version of MOXIE is small by design, in order to fit aboard the Perseverance rover, and is built to run for short periods, starting up and shutting down with each run, depending on the rover’s exploration schedule and mission responsibilities. In contrast, a full-scale oxygen factory would include larger units that would ideally run continuously.
More detailed report.