Simply resetting the ECU after replacing the O2 sensor should NOT cause starting running problems. drop a 1-2 mpg for a hundred miles yes, actually run rough no.
check for things like vacuum leaks or broken wires (that you mighta caused fixing things)
__________________
-Russell
1991 Toyota Pickup 22R-E 2.4 I4/5 speed
1990 Toyota Cressida 7M-GE 3.0 I6/5-speed manual
mechanic, carpenter, stagehand, rigger, and know-it-all smartass
"You don't get to judge me for how I fix what you break"
Mr. Pig 
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