The A320 has fixed/cyclinder O2 for the flight deck, chemical O2 for the pax in the overhead compartment and portable O2 cylinders for the cabin crew: https://aviationthrust.com/airbus-a3...k-airbus-a320/

So it may be that the portable O2 kit is stored in the overhead thingie spotted by the OP instead of blocking some of the overhead bins.