ACR FLT CREW VOICED CONCERN REGARDING ATC SERVICE AT ORD WHEN LCL CTLR APPARENTLY CONFUSED TWO DEP ACFT AWAITING TKOF CLRNC.

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WX AT ORD WAS DARK; CLOUDY; RAINY; AND THERE WERE MANY DELAYS (US INCLUDED) TO THE E COAST DUE TO LOW VISIBILITY AT OUR DEST. WE HAD HELD FOR APPROX 1 HR; AND THEN WERE INSTRUCTED TO TAXI TO RWY 9R VIA TXWYS G; Z; AND J. WE WERE ON TXWY J HOLDING SHORT OF RWY 9R ON THE N SIDE OF THE RWY. AN ACR Y ACFT WAS ON THE S SIDE OF RWY 9R AROUND TXWY J. ATC GAVE ACR Y INSTRUCTIONS TO POS AND HOLD RWY 9R; TFC 5 MI FINAL RWY 9R. ACR Y SAID THAT THEY NEEDED TO GO BACK TO THE GATE. ATC THEN ASKED ACR X IF WE WERE READY; AND MY COPLT ACKNOWLEDGED THAT WE WERE READY. ATC THEN GAVE ACR X INSTRUCTIONS TO POS AND HOLD RWY 9R. AS WE ENTERED THE RWY; AN ACR Z QUERIED ATC AS TO WHO WAS SUPPOSED TO BE POS AND HOLD ON RWY 9R. ATC REPLIED THAT ACR Z WAS SUPPOSED TO BE IN POS AND HOLD RWY 9R. MY COPLT IMMEDIATELY TOLD ATC THAT WE WERE ENTERING THE RWY POS AND HOLD. ATC THEN TOLD US TO EXIT THE RWY; AND WE TAXIED QUICKLY DOWN RWY 9R AND TURNED L ONTO TXWY E AND EXITED THE RWY. THE SAME ATC CTLR WAS WORKING TKOFS ON RWYS 32R; 4L; 9R; LNDGS ON RWY 9R AND XING TAXIING ACFT ACROSS RWY 9R. HE WAS OBVIOUSLY OVER-TASK SATURATED. I TRULY BELIEVE THAT ATC GAVE ACR X THE INSTRUCTIONS TO POS AND HOLD RWY 9R AND THIS WAS NOT A 'HEARBACK/READBACK ERROR.' I BELIEVE THAT BECAUSE THE ATC CTLR WAS OVER-TASKED; HE GAVE ACR X THE INSTRUCTIONS TO POS AND HOLD; THINKING THAT HE MIGHT HAVE GIVEN THE INSTRUCTIONS TO ACR Z. AFTER HE REALIZED THAT ACR X WAS GOING INTO POS AND HOLD; HE ASKED US TO CLR THE RWY. WE TAXIED ON TXWYS E; Z AND J BACK ON THE N SIDE OF RWY 9R AND HELD SHORT OF THE RWY. AFTER ATC GAVE US OUR INSTRUCTIONS AGAIN TO POS AND HOLD RWY 9R; WITH TFC 5 MI FINAL; WE SAT ON THE RWY IN POS WHILE HE INSTRUCTED TFC TO DEPART RWYS 4L; 32R; AND PUT ANOTHER ACFT INTO POS AND HOLD RWY 4L. IT WAS VERY UNNERVING THAT WE WERE IN POS AND HOLD SO LONG (WELL OVER 1 MIN) WHILE TFC WAS BEHIND US LNDG ON THE SAME RWY THAT WE WERE HOLDING ON. AFTER TFC DEPARTED RWY 32R; WE WERE GIVEN OUR TKOF CLRNC; AND DEPARTED SAFELY. I UNDERSTAND THAT ORD ATC CTLRS ARE THE BEST IN THE BUSINESS; BUT THIS EVENING; THIS CTLR WAS OVER-TASKED BY WORKING SO MANY DIFFERENT RWYS.