AN ACR FO DIALED IN ORD TWR FREQ VICE THE REQUIRED TRACON APCH CTL FREQ.

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ON APCH TO ORD RWY 4R ON R DOWNWIND; WE WERE TURNED ON BASE LEG. AT THAT POINT THE FO PLACED TWR FREQ OVER THE APCH CTL FREQ. WE CALLED TWR THINKING IT WAS STILL APCH AND WAS TOLD TO RETURN TO APCH. WE NO LONGER HAD THE PROPER FREQ AND WE CROSSED THE APCH PATH FOR RWY 4L. WE ASKED THE PREVIOUS APCH CTL AND TWR FOR THE PROPER FREQ AND RETURNED TO THE APCH APCH FOR RWY 4R. WE CALLED TRACON ON THE GND AND THE MISHAP WAS VERBALLY RESOLVED. SUPPLEMENTAL INFO FROM ACN 303298: WHILE BEING VECTORED FOR AN ILS TO RWY 04R AT O'HARE; THE PNF; COPLT; SWITCHED WHAT I (THE COPLT) THOUGHT WAS THE NON-SELECTED RADIO FREQ FROM APCH CTL TO TWR FREQ. I WAS PLANNING AHEAD; SETTING THE TWR FREQ IN THE RADIO. I INADVERTENTLY SET THE TWR IN THE OPERATING SIDE OF THE RADIO. WE THEN FLEW THROUGH THE FINAL APCH COURSE AND ABOUT 2 MI N OF THE APCH COURSE BEFORE WE CALLED TWR - STILL THINKING WE WERE TALKING TO APCH CTL. HE SENT US BACK TO APCH CTL. WE CONTACTED HIM (AFTER CALLING THE PREVIOUS CTLR) AND WERE VECTORED BACK FOR AN APCH AND LNDG. MY MISTAKE.