AN ACR MD80 FLC STARTED THEIR APCH DSCNT REFING THE WRONG DME SOURCE BECAUSE THE FO FORGOT TO CHANGE NAV FREQ.

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ILS RWY 22R ORD WAS BRIEFED. CAPT WAS FLYING AUTOPLT ON CAPT SIDE. WE WERE CLRED TO OBK TO INTERCEPT LOC RWY 22R GS OTS. I WAS BRIEFING LOC RWY 22R APCH WHEN WE WERE CLRED FOR APCH AND CROSS FNUCH 7000 FT OR ABOVE. VERY SHORTLY AFTER; THE ARMED LOC CAPTURED. WE WERE STILL BRIEFING WHEN I NOTICED THE DME FOR STEPDOWN. WHAT I FAILED TO REALIZE WAS THAT FO DID NOT SWITCH TO ORD FOR DME. DSCNT WAS EARLY BY A FEW MI. WE WERE GIVEN SPD REDUCTIONS WHICH KEPT US A LITTLE HIGH ON STEPDOWNS TO KEEP US FROM DEVIATING AT NOLEN WHEN ERROR WAS DISCOVERED. OTHER STEPDOWNS AND LNDG WERE NORMAL. CONTRIBUTING FACTORS WOULD BE OUR BRIEFING THE NEW APCH WHILE INTERCEPTING AND GETTING THE APCH CLRNC IN A VERY SHORT TIME FRAME. SUPPLEMENTAL INFO FROM ACN 433000: WE MISTOOK OBK DME FOR ORD DME AND BEGAN DSCNT FROM 7000 FT TO 5000 FT PREMATURELY. CAPT (PF) REALIZED MISTAKE AND WE CHANGED TO ORD FREQ. BECAUSE OF DELAY IN DSCNT WITH SPD REDUCTIONS; NO ALT BUSTS MAY HAVE OCCURRED. WHILE THERE IS NO EXCUSE FOR MY MISTAKE; THERE WERE SOME CONTRIBUTING FACTORS. THESE WERE OFF RTE VECTORS TO INTERCEPT RWY 22R LOC WITH INTERCEPT OCCURRING AT 19.5 DME ON OBK VOR WHICH IS A XING POINT ON THE ORD VOR; SIMILAR DME DISTANCES AND FREQS OF 2 NEARBY VORS ORD 113.9 AND OBK 113.0; AND LAST MIN NOTIFICATION OF GS INOP AND CHANGE TO LOC FREQ.