ok...there aren't any drop down masks in a little C421.
I imagine, if you made a choice to use oxygen, there are small outlets and you would have to plug your mask in yourself...this for the passengers
as for the crew...I imagine you could get a quick donning pressure mask. but you might have to pay extra for it, otherwise just a regular little cup maske.
now...please remember the HELIOS AIR crash near ATHENS GREECE in augusut of 2005...the warning horn sounded for excess cabin altitude but the crew thought it was the gear warning (they can sound similiar sort of) but didn't recognize the need to go on oxygen.
it is easy to explain this as an accident, hypoxia, subtle depresurization, or medical problem for the good doctor flying the plane.