BECAUSE RWY 27L AND RWY 9R ARE THE DESIRED RWYS FOR ACFT GOING TO THE CARGO RAMP; THE DC8-55 CREW ANTICIPATED AND PLANNED RWY 27L EVEN THOUGH THEY WERE APCHING FROM THE N. THEY WERE ALIGNED WITH RWY 27L WHEN THE TWR ADVISED THEM THEY SHOULD HAVE BEEN ON THE RWY 27R APCH. ACFT WAS VECTORED TO THE PROPER APCH PATH.

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APCH PATH INCURSION TO RWY 27L AT ORD. ORIGINAL CONTACT WITH ORD APCH CTL ON 119.0 16 DME W OF KRENA INTXN FOLLOWING LOSS OF CONTACT WITH ZAU. LAST INSTRUCTION WAS TO MAINTAIN 320 KTS AND LEVEL AT 10000 FT TO KRENA. APCH CTL SUBSEQUENTLY INSTRUCTED A DSCNT TO 7000 FT. WE FOLLOWED BY SLOWING TO 250 KTS THEN BEGAN THE DSCNT. THE FO WAS FLYING TO WHAT WE EXPECTED WAS APCHS TO RWY 27L AND HAD BRIEFED THE APCH FOR THIS RWY. LEVELING AT 7000 FT; THE FO NOTED A DIFFERENCE IN HORIZON PITCH REF OF 5 DEGS. THINKING THE ADJUSTMENT KNOB HAD BEEN MOVED; I ASKED HIM TO ADJUST THE HORIZON BALL TO COMPARE WITH THE CAPT'S INDICATOR AND THE STANDBY HORIZON INDICATION. APCH CTL GAVE FURTHER CLRNC TO DSND AND MAINTAIN 4000 FT AND SLOW TO 210 KTS. APCH FREQ WAS SET FOR RWY 27L. BELIEVING THAT CLRNC WAS GIVEN FOR RWY 27L WE PREPARED FOR THE APCH. CLRNC WAS GIVEN TO TURN TO HDG 180 DEGS; MAINTAINING 4000 FT AND SLOW TO 170 KTS. FURTHER CLRNC WAS GIVEN TO MAINTAIN 4000 FT AND HDG 250 DEGS TO INTERCEPT THE APCH COURSE. THAT'S ALL I HEARD; BECAUSE AT THE SAME MOMENT THE FO WAS SPEAKING ABOUT A GYROSCOPE PROB AND THAT THE HORIZON INDICATOR WAS BEGINNING TO TUMBLE. THE CAPT TOOK CTL TO COMPLETE THE TURN TO 250 DEGS AND FLY THE APCH. APCH CTL THEN INSTRUCTED US TO CHANGE TO TWR FREQ. THE FO READ BACK THE INSTRUCTION THEN CHANGING TO THE FREQ GIVEN; BUT FOUND IT WAS A FREQ OTHER THAN TWR. HE ASKED ME AS WE INTERCEPTED THE LOC WHAT THE TWR FREQ WAS FOR THE RWY. I SAID 120.75 FOR THE S SIDE. UPON TUNING THIS FREQ; THE TWR SAID DSND IMMEDIATELY TO 3000 FT AND TURN R TO INTERCEPT RWY 27R APCH. AT THIS POINT WE WERE STRUCK WITH THE REALIZATION THAT WE INTERCEPTED THE WRONG APCH PATH. I WAS STUNNED BY WHAT COULD HAVE OCCURRED; BUT LANDED VFR ON RWY 27R. BROKEN CLOUD LAYERS ABOVE 3200 FT PREVENTED US FROM CLRLY SEEING OTHER TFC; BUT WE SAW ACFT LIGHTS IN SIGHT WELL AHEAD AND FURTHER OUT AS WE TURNED TO FINAL. I THINK THE LOSS OF CONTACT WITH ZAU CAUSED SOME DISTRESS TO DEVELOP FOLLOWED BY ATTENDING TO SLOWDOWN AND INCREMENTAL DSCNT; PLUS ATTN DIVERTED TO THE HORIZON INDICATOR CONCERNS AS TO WHICH SIDE WAS FAULTY; COUPLED WITH THE BUSY FREQ ON APCH LED US TO CHANNEL OUR ACTIONS TOWARD LNDG ON RWY 27L AS WE PREVIOUSLY BRIEFED. I BELIEVE THAT HAD WE KNOWN OR WERE CLRLY AWARE THAT SIMULTANEOUS APCHS WERE BEING CONDUCTED TO PARALLEL RWYS THE PROCS FOR BOTH RWYS 27L&R WOULD HAVE BEEN THOROUGHLY REVIEWED AND PREPARED FOR EITHER RWY. MY METHOD OF PLANNING IN THE FUTURE WILL BE TO REVIEW AND PREPARE FOR THE POSSIBILITY OF APCHS TO EITHER ONE OF PARALLEL RWY WHEN SIMULTANEOUS APCHS ARE IN OP.