As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CAPTAINS O2 MASK MAIN HOSE CAME OFF DURING PREFLIGHT SYSTEM CHECK. SWAP PILOTS O2 MASK W/4TH OBSERVER MASK OP CHECKS NORMAL AIRCRAFT OK TO CONTINUE. 4TH OBSERVERS O2 MASK INOP. DEFERRED PER MEL 25-11-07A CT# 1154512 DUE DATE 07-07-2020.