B767-300 CREW AND AN MD80 ALMOST COLLIDED ON THE GND AT ORD.

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WE LANDED AT ORD ON RWY 9R AT NIGHT; (ABOUT 2200 FT) EXITED L ON TXWY M5; CLRED BY TWR TO TAXI VIA TXWY A TO OUR GATE ON FREQ 120.75. TO GET TO TXWY A; THE NORMAL ROUTING FROM TXWY M5 WOULD BE W ON TXWY D; TO TXWY A. WHILE APCHING TXWY A ON A16; (WHICH IS NOT A 90 DEG INTXN) WE HAD TO MAKE A VERY QUICK STOP TO AVOID AN MD80 TAXIING W ON TXWY A APCHING A16 FROM THE E (AROUND A18). WE WERE NOT ADVISED OF ANY ACFT THAT WOULD BE APCHING FROM OUR R REAR; WHICH IS WHERE THAT ACFT WAS. THE LOCATION OF THAT ACFT ON TXWY A SHOULD HAVE ALLOWED IT TO STOP FOR US BECAUSE WE WERE RIGHT IN FRONT OF ITS WINDSCREEN ON A16; IT WAS NOT IN FRONT OF US ON TXWY A; AS BOTH ACFT WERE FACING MOSTLY W; THE MD80 SLIGHTLY BEHIND US ON TXWY A. HOWEVER; BECAUSE THAT ACFT WAS APPARENTLY ON GND CTL FREQ; NOT TWR FREQ; IT MAY NOT HAVE BEEN ADVISED OF OUR POS AS WE WERE NOT ADVISED OF ITS. TWR CTLR CAME ON THE RADIO AND APOLOGIZED FOR THE CONFLICT BECAUSE HE KNEW WE HAD TO STOP QUICKLY TO AVOID A COLLISION. MY FO SPOTTED MOVEMENT OUT HIS SIDE OF OUR AIRPLANE AND ADVISED ME TO STOP WHICH I DID ON A16; IN TIME TO WATCH THE MD80 CONTINUE TO TAXI W ON TXWY A MUCH TOO CLOSE FOR COMFORT; WHICH IS WHAT HAS PROMPTED THIS RPT. WHILE DEPLANING; MANY OF OUR PAX INDICATED THEY HAD NOTICED THE CLOSENESS OF THE 2 AIRPLANES. THIS INCIDENT IS ALL ABOUT TAXIING ON SPLIT FREQS DURING LATE EVENING (NIGHT) HRS WHEN PROCS (WITH JUST A FEW ACFT MOVING ON THE ARPT) ARE HANDLED (WHAT SEEMS TO BE) A BIT MORE INFORMALLY. BECAUSE OF THE LATE HR; WE SUSPECTED THE MD80 WAS ON ITS WAY TO THE HANGAR AND POSSIBLY BEING OPERATED BY MECHS; WE DO NOT KNOW; IT DID NOT STOP. OBVIOUSLY IT IS THE BEST POLICY TO SWITCH TO GND CTL FOR TAXI CLRNCS; BUT IF A TAXI CLRNC IS OFFERED BY THE TWR CTLR WE NORMALLY ACCEPT IT. WE WERE VERY; VERY LUCKY WE HAD AN ALERT FO THIS EVENING. HE SAVED EVERYONE A TAXI ACCIDENT. THESE 2 AIRPLANES WOULD HAVE TAXIED INTO EACH OTHER AT A16 AND TXWY A IF NOT FOR HIS ALERTNESS.