CRJ-900First Officer reported the crew oxygen mask failed a First Flight of the Day preflight check. The expanding net that goes over your head was broken. The rubber piping that expands had come apart inside of the nylon casing that protects the rubber. The aircraft was removed from service for repair.

Narrative

I got to the plane for our first flight of the day; but the plane had already flown already. During my normal First Officer preflight check my oxygen mask failed its check. I always do a FOD check on the mask; minus the speaker when I get into a new plane for the day. When checking the quick don function (the pinch on the mask) it kept flowing and didn't stop. I pulled it out and realized the expanding net that goes over your head was broken. The rubber piping that expands had come apart inside of the nylon casing that protects the rubber. If I needed this at altitude; the quick donning function wouldn't work and it would not hold to my face. This is the second mask in a few months that I found that failed its preflight check. Being the second mask; I've found like this; I think the quick donning net that goes over your head needs replacement more often. I also think this mask hasn't been properly checked in months. I would re-educate and reiterate the importance of a proper mask preflight via memo.