Or use an air compressor at low psi !!!
Atmospheric air being only about 20% O2 would need more time, but plenty of brewers use it. Tried and True.
if you go this route.... definitely use one of the disposable sterile filters 0.2 micron at low pressure. For an air compressor I would even think about replacing it after each use. With an aquarium pump it will also serve to remove any airborne particulate.
Bacteria ranges from 1-10 microns, I have serious doubts about the efficiency of the disposable filters but they are better than nothing. I specialize in compressed gas treatment, compressed air will be wet and dirty and can provide an environment for bacterial or fungal growth especially in a tank-mounted unit (think dark, wet, plenty of O2).
I notice green O2 tanks online every now and then, probably a home breathing apparatus but would be very nice to have for brewing. Judging by the growing pile of red cylinders I have in my garage I might go this route myself. When I empty 1 red cylinder I start shopping for 2 more.