B737-200 CREW DID NOT DETECT THAT CREW AND PAX OXYGEN WERE NOT TURNED ON AFTER A MAINT CHK.

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I FLEW A B737-200 AFTER A MAINT CHK. IT WAS AN EARLY MORNING DEP AND WE WERE BOTH FEELING THE EFFECTS OF WAKING UP EARLY. I GOT UP IN THE MIDDLE OF MY PREFLT FLOW TO GET MORE COFFEE AND DISTRACTED MYSELF FROM MY USUAL CAREFUL CHK OF THE OXYGEN MASK. WHAT ARE THE CHANCES THAT MAINT AND THE CREW WOULD MISS A DEACTIVATED OXYGEN SYS ON THE SAME DAY? INCREDIBLY; WE DID! ALSO DISTURBING WAS MY NOT SEEING LOW INDICATIONS ON THE OXYGEN GAUGES. INDICATIONS THAT ASSUREDLY WERE EVIDENT AND MISSED BY ME ON OUR 7 LEG DAY! MY HABIT IS TO POINT TO MANY GAUGES AS I LOOK AND THEREFORE WAS SURPRISED THAT I FAILED TO CATCH A PROB ON THIS DAY. FACTORS: DAYLIGHT SAVINGS TIME -- DIDN'T GET ENOUGH SLEEP. SUPPLEMENTAL INFO FROM ACN 579174: EVENT OCCURRED ON THE FIRST DAY OF A 3-DAY PAIRING. IT WAS A 7 LEG B737-200 DAY WITH A XA30 CENTRAL RPT. THE NIGHT PRIOR; MAINT HAD REPLACED ONE OF THE HYD PUMPS ON THE ACFT. DURING THIS PROC; THEY DEACTIVATED BOTH THE PAX AND CREW OXYGEN SYS. WE WENT THROUGH WHAT WE THOUGHT WAS A NORMAL PREFLT AND ON-TIME PUSH. AFTER TURNING ON THE 'A' SYS PUMPS; WE STILL HAD A LOW PRESSURE LIGHT IN THE ENG DRIVEN PUMP. WE TAXIED BACK TO THE GATE AND LET MAINT RE-CHK THE HYD SYS. THEY DETERMINED THAT THE SYS HAD AIR IN THE LINE AND; BY ACTIVATING THE CTL SURFACES SEVERAL TIMES THEY WERE ABLE TO PURGE THE AIR FROM THE LINES. WE PUSHED ABOUT 30 MINS LATE AND WERE ON OUR WAY. SOMEHOW; DURING OUR PREFLT DUTIES; WE FAILED TO PROPERLY CHK THE OXYGEN SYS. I SET THE MASK UP TO FIT ME AND TURNED THE SYS ON AND WENT TO 100% AND EMER AND THOUGHT I HAD FLOW. I DO NOT REMEMBER CHKING THE OVERHEAD GAUGES AND CANNOT REMEMBER DOING ANY OF THE REST OF THE CHK. I MAY HAVE BEEN INTERRUPTED IN MY FLOW; BUT CAN'T SAY FOR SURE. WE FLEW THE REST OF THE DAY AND FAILED TO RECOGNIZE ANY PROBS. WE PLAYED CATCH-UP ALL MORNING AND I DO NOT BELIEVE I CHKED THE OXYGEN AGAIN. WE GAVE THE AIRPLANE UP AND THE NEW PLTS NOTICED THAT THERE WAS NO FLOW ON THE MASKS. CREW OXYGEN GAUGE READ 200 AND PAX GAUGE 300. I BELIEVED THAT FATIGUE WAS A CONTRIBUTING FACTOR ON MY PERFORMANCE EARLY IN THE MORNING. BUT MY POOR PERFORMANCE DURING THE PREFLT; AND SUBSEQUENT LEGS; WAS DUE MOSTLY TO COMPLACENCY. I JUST FAILED TO DO WHAT I WAS SUPPOSED TO DO. I MADE NO CONSCIOUS DECISION TO OMIT THIS ITEM; BUT TIME AND AGAIN; I FAILED TO CHK THE OXYGEN. THE LESSONS I HAVE LEARNED ARE HOW IMPORTANT EVERY CHK CAN BE AND TO NEVER ALLOW YOURSELF TO SKIP OR OMIT ANY ITEMS. THE CHKS WE PERFORM ON A ROUTINE BASIS ARE ANYTHING BUT ROUTINE. I WILL ALSO TREAT AN ACFT COMING OUT OF MAINT WITH MUCH MORE CAUTION AND DOUBLE-CHK EACH ITEM.