The 0² sensor detects variations in exhaust gas content and sends a correction signal to the FI control box, which then adjusts mixture. This is done on a continuing basis. Thus it is normal for an FI engine to "hunt" at idle -- change RPMs slighly up & down as the system constantly adjusts. This is a sign of a working controller & O² sensor.
However it should not hunt excessively; maybe 50 RPM or so. If it's swinging several hundred RPM, you have other more serious problem. Likewise if it changes idle RPM a large amount after warming up.