POTENTIAL CONFLICT BETWEEN DEPARTING ATX AND ARRIVING GA TWIN. OCCURRENCE DOWNGRADED FROM OPERATIONAL ERROR.

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ATX X WAS HOLDING IN POSITION ON RWY 27L. TFC Y WAS 8-10 MILES SW OF THE ARPT EBOUND AT TWO THOUSAND FOR RWY 35. X WAS ADVISED THAT HE WAS WAITING FOR ACFT Y TO CLEAR A LINE FROM THE ARPT DIRECT OOD VOR. ACFT X WAS CLRED FOR TKOF AND GIVEN A TURN DIRECT OOD FOR WEATHER AVOIDANCE. (THERE WERE LEVEL 5-6 THUNDERSTORMS WEST TO NORTH OF THE ARPT). AT THIS POINT ACFT Y CONTACTED ME ON THE LOCAL FREQ AND WAS TOLD TO SQUARE HIS BASE TO FINAL TO FURTHER KEEP HIM CLEAR OF X'S FLT PATH. WITHIN SEVERAL SECONDS I VISUALLY; AND ON THE BRITE; OBSERVED ACFT Y TURNING NBOUND INTO ACFT X. I VISUALLY OBSERVED THE ACFT CONVERGING AND ISSUED AN IMMEDIATE TURN WBOUND TO X AND A TURN EBOUND TO Y. I CALLED TFC TO BOTH ACFT AND WHEN ACFT Y REPORTED THE OTHER IN SIGHT I INSTRUCTED HIM TO MAINTAIN VISUAL SEPARATION. I THINK A CONTRIBUTING FACTOR TO THIS SITUATION WAS THAT ACFT Y MOST LIKELY DID NOT HAVE RWY 35 IN SIGHT WHEN HE WAS CLRED FOR THE APCH AND TURNED OVER TO THE TWR. THIS CAUSED HIM TO START A TURN TO FINAL WHEN HE WAS STILL SEVERAL MILES SW OF THE ARPT. ALSO; LOCAL HAD COORDINATED FOR ALL DEPS TO PRECEED DIRECT OOD AND THIS WAS EITHER FORGOTTEN BY THE OOD SECTOR CTLR OR IT WAS NOT INCLUDED IN A BRIEFING DURING CTLR CHANGE. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING INFORMATION: CLARIFIED TIME OF INCIDENT AS AT NIGHT. RPTR CANNOT RECALL OFFICIAL VISIBILITY; BUT COULD SEE ACFT Y LIGHTS AT 5 MI FROM ARPT. CLARIFIED TYPE OF ACFT X. GOT YEARS OF ATC EXPERIENCE FOR LCL CTLR AND APCH CTLR. AFTER REVIEW WITH ATC SUPERVISORS INCIDENT WAS DOWNGRADED FROM OP ERROR BECAUSE LCL CTLR HAD BOTH ACFT IN SIGHT AND WAS ABLE TO EFFECT VISUAL SEP. STILL NOT SURE IF THERE WAS A BRIEFING OF RELIEF PROBLEM IN TRACON. NORMAL DEP PROC IS HDG 255 DEG OFF RWY 27 UNTIL DEP CTLR TURNS TO OOD VOR. PLTS DID NOT WANT TO FLY INTO TSTM ACTIVITY SO WERE BEING GIVEN DIRECT OOD BY LCL CTLR. ACFT HAD APPROX 300' ALT SEP AT CLOSEST POINT. THINKS PLT OF Y MADE TURN TO NORTH TOO SOON BECAUSE HE SAW WELL LIGHTED ROADWAY WHICH IS ABOUT SAME LENGTH AS RWY 17/35 AND RUNS PARALLEL.