Installed a new O2 sensor, Is there any break-in time required ?
Need to take the car in for Smog test ...

No, but the ECM will need a day or two of driving above 45 MPH ( I believe) to readjust for the new input information that it will now be recieving from a part that will more than likly be functioning differently that the old part.
As an O2 sensor gets older carbon will build up and though it is working, it is not getting a proper reading (reason you should change them every so often). A new O2 will give proper readings but the ECM will have changed certain perameters for the old O2 sensor to compensate for it starting to fail, so now it will have to readjust to the new and correct input.
If you have a V6 there is a MAF sensor on the housing to the aircleaner, the part is under $10.00 ( last time I changed one 4 years ago ) and a good idea to swap that out at the same time.
MAF reads how much air is comming into the engine. O2 reads how much is left in the exhaust. If the MAF is old it may give false readings to the O2 and the ECM usualy throws a code of a bad O2. The reason is the ECM sees the MAF as "working" even if is not working very well, ECM only gives you a code when the MAF does not work at all, again, good reason to get a new one. Not sure if the 4 bangers have a MAF on them, been over 10 years since I owned or worled on one.

quote Originally posted by OH10fiero: No, but the ECM will need a day or two of driving ... to readjust ...

That is essentially correct.

quote If you have a V6 there is a MAF sensor on the housing to the aircleaner ...

No!!! There is no MAF (Mass Airflow) sensor present in any stock Fiero. Instead, the V6 Fieros use a TPS (Throttle Position Sensor), MAP (Manifold Absolute Pressure) sensor, and an IAT (Intake Air temperature) sensor, along with engine RPM, to form a speed-density fuel control system. The MAP and IAT sensors allow calculation of the density of air in the intake plenum. Changes in the TPS output are used by the ECM to estimate transient (i.e. very short term) changes in intake air density.

[This message has been edited by Marvin McInnis (edited 05-07-2010).]

The Duke or 4 banger does not have a MAF or mass airflow sensor . I believe the ECM uses speed density for A/F measurement.
The ECM was reset after installing the O2 sensor, so a few days with speeds above 45 the new sensor should be good.

edt: Marvin - You have the fast keyboard tonight, or should I say this morning.

[This message has been edited by StockGT (edited 05-07-2010).]