First Officer reported failure to change oxygen tank resulted in confusing communications efforts with ZMA ARTCC and expeditious descent.

Narrative

Enroute from ZZZZ to Palm Beach International Airport at FL 180. Pilot and First Officer (Self) are returning from a two day cargo run. The twin turbine aircraft we were flying is not pressurized and requires us to breath supplemental oxygen using cannulas or masks attached to a regulator and oxygen tank. During flight; our oxygen tank was depleted and required us to change to a different tank. The regulator on the depleted tank had to be removed and placed on the replacement tank. This normally takes about 1 minute. I was the flying pilot and the Captain offered to switch the tank. Unable to locate a wrench to remove the regulator from the spent tank; the Captain tried to twist the regulator off by hand. This resulted in the regulator being in two pieces; the top half came off the tank and the bottom half did not. Unable to locate a wrench to remove the bottom half of the regulator from the empty oxygen tank; we could not attach the regulator to the replacement tank. I was wearing an oxygen mask with the comms microphone installed in it. The mask's oxygen reservoir bag was used up and I could not breath wearing the mask. When I pulled it away from my face to breath; the microphone was too far away from my lips for ATC to hear me. I called ATC to let them know I needed an immediate descent. After a short delay; they queried; 'who was that calling for an immediate descent?' I let them know our tail number and again asked for immediate descent but they could not hear me because I had to pull the microphone away from my face to be able to breath. At this point; I could see that our oxygen issue was not going to be resolved in a known amount of time and I was not able to communicate with ATC or my Captain and breath at the same time; so I pulled the throttles back and started an aggressive descent to 12;000 feet. At this point; the Captain took over comms with ATC and let them know what was going on. There seemed to be a lot of confusion on the radio as to what was going on.