AS 1ST OFFICER (FO) WAS FLYING ACFT, DRG CLB AFTER TKOF, CABIN WOULDN'T PRESSURIZE. HE BGN LVL OFF AT 16,000', BUT CAPT ORDERED HIM TO CONT CLBG TO ASSIGNED FLT LVL (FL 330), WHILE HE (THE CAPT) WENT AFT TO FND PRBLM. FO DISLIKED THE ORDER,BUT COMPLIED RATHER THAN CONFRONT CAPT. CAPT LEFT COCKPIT WITH PORTABLE 'ON DEMAND' OXYGEN (O2) SYS, WHICH HAD 15 MIN SUPPLY OF O2. WHEN CAPT DIDN'T RTRN, FO TRIED SIGNALING HIM. THO RELUCTANT TO COUNTERMAND CAPT'S ORDER, FO MADE SERIES OF DSCNTS TO 13,000'. AFTER ABT 30 MIN, HE LEFT COCKPIT & FND CAPT UNCONSCIOUS & UNRESPONSIVE IN FWD CARGO AREA WITH O2 MASK ON HIS FACE. CAPT'S FOOT WAS ENTANGLED IN CARGO NET WHICH CVRD A PALLET. FO TRIED TO REVIVE CAPT, TO NO AVAIL, THENDECLARED EMERG & LNDD AT LUBBOCK. CAPT WAS RUSHED TO HOSPITAL, BUT WAS DEAD ON ARRIVAL. EXAM REVEALED AFT PRESSURE BULK-HEAD WAS NOT INSTALLED. IT HAD BEEN REMOVED FOR MAINT BFR FLT. PORTABLE O2 SYS WAS STILL FULL, INDCG CAPT HAD LITTLE OR NO USE OF O2; IT WAS TESTED & FUNCTIONED NMLY. GROUP OF 8 FORENSIC PATHOLOGISTS CONCLUDED CAPT DIED FM HYPOXIC HYPOXIA.