There are really multiple variable here. First one must not make the mistake on confusing barometric pressure with the partial pressure at which O2 (PPO2) is being delivered. For O2 gas exchange in the lungs there must be adequate PPO2 which IIRC is about .05 atm anything lower and the O2 will not diffuse into the blood stream.

For example, breathing 100% O2 at sea level (1 atm) is the same as breathing air (~20% O2) at 40 meters under the water (~5 atm). In both case the PPO2 is 1 atm (100% x 1 atm == 0.20 * 5 atm).

One can go the other way and breath 100% O2 at 8000 meters (~1/3 atm) which would be the same as breathing 30% O2 at sea level.

So if you want to simulate cabin pressure (~2500 m) which about .75 atm. Get a cylinder of gas composed of about 15% O2 and 85% nitrogen and breath that using a full face mask.

Of course remember the mask is not going to be a perfect seal so as said above the only way to get it "exact" would be in a pressure vessel.