CAPT OF AN EMBRARER EMB120 ABORTED THE TKOF AFTER HEARING AN UNUSUAL NOISE IN THE COCKPIT. POSTFLT INSPECTION REVEALED THAT THE JUMP SEAT OXYGEN MASK HAD FALLEN OUT OF ITS HARNESS AND FELL ON THE 'PRESS TO TEST BUTTON' RELEASING OXYGEN FROM THE MASK.

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I WAS CALLED IN ON A RESERVE DAY TO DO A ROUND TRIP BTWN CLE AND BDL; CT. I WAS PAIRED WITH A VERY NEW FO WHO I HAD NEVER FLOWN WITH BEFORE. EVERYTHING WAS NORMAL UP TO OUR TKOF ROLL. UPON APPLYING TKOF PWR; I HEARD A DISTINCT NOISE OF SOMETHING THAT HAD FALLEN BEHIND THE FO'S SEAT. A FEW SECONDS LATER I HEARD A LOUD HISSING NOISE THAT I RECOGNIZED AS COMING FROM THE OXYGEN SYS. I ABORTED THE TKOF AT APPROX 60-70 KTS. WE INFORMED TWR THAT WE HAD ABORTED AND TAXIED BACK TO THE PARKING AREA. IT WAS HERE THAT I NOTICED THE OBSERVER'S QUICK-DONNING OXYGEN MASK HAD FALLEN OUT OF ITS HARNESS AND ITS PRESS-TO-TEST BUTTON HAD BEEN DEPRESSED DUE TO THE WAY IT HAD FALLEN. INITIALLY; I THOUGHT THE FO'S SEAT HAD SLID BACK; BUT THIS WAS NOT THE CASE. I CALLED A MAINT PERSON OUT TO THE PLANE TO ENSURE THAT THE MASK WAS STORED PROPERLY IN ITS HARNESS AND WE CONTINUED THE FLT AFTER BEING RERELEASED BY DISPATCH TO BDL WITHOUT FURTHER INCIDENT. NOW; EVERY TIME I ENTER THE BRASILIA; I MAKE SURE THAT THE OBSERVER'S MASK HARNESS IS FITTED TIGHTLY AROUND THE MASK.