Air carrier pilot reported multiple flight deck oxygen masks were missing required components.

Narrative

We arrived at aircraft and after completing my flows; I attempted to clean my oxygen mask as history has proven they are normally rather dirty and lint covered. I found the head of rivet at the bridge of the full face mask nose was missing. I then checked the 1st observers mask to verify its condition; and found the same problem with the mask. I then asked the FO to look at his mask. It had the exact same issue with the rivet head being broken off. Only the second observer's mask was fully operational. I contacted Dispatch and Maintenance Control to report the situation. Multiple full face oxygen masks broken.Suggestions: The Company needs to have either or both Maintenance and the Pilot group clean and inspect the oxygen masks on the first flight of the day. When the Pilots were required to wear their oxygen masks when the other Pilot left the cockpit at altitude they normally clean and inspected the oxygen masks. Since the FAR change during COVID; primarily only Maintenance inspects the mask and they are only verifying that the retention harness is inflating within 3-seconds. Pilots are more likely to encounter a pressurization/smoke or fume event than an engine failure during their career; so both Flight Ops and Maintenance should ensure that the oxygen masks are fully functional. The Company should contact manufacturer to find if the mask is capable of dealing with a smoke/fume event or a depressurization event with the head of the rivet and/or if the rivet is completely missing. If so; then add an MEL so we can use the oxygen mask safely to get to a base or airport that it can be fixed or replaced.