Nissan Repair: 02 Altima Rough Idle, oxygen sensor, o2 sensor

Question
I have recently had some work done.  I had two gaskets replaced that was causing an oil leak (I was replacing about a quart of oil every 350 miles).  They replaced the gaskets to resolve the oil loss and the spark plugs that was causing a stalling issue (they were replaced by a repair shop in August of 07 but apparently they put in incorrect spark plugs at the time of that repair).  Now I am having a rough idle issue.  When coming to a stop, whether I am idling in drive or in park, the tach will fluctuate between 400RPM's and 900RPM's.  When it dips down to 400RPM's, it sounds and feels like the engine could stall again (but hasn't yet).  I was told that I have a bad oxygen sensor, and this is causing my car to run rich and may cause rough idle, but are the symptoms I am describing and seeing typical with a bad O2 sensor?  Could there be another issue that they are overlooking and I need to be worried about?  Thank you for any information you can provide.  I appreciate your help.

Answer
Daniel,

It is an easy thing to replace the sparkplugs first.  Then, if you still have problems like a check engine light go down to Autozone or Checker and have them read the code for free and email me back with what they tell you.  Most of the time it is a simple part like an oxygen sensor and it is just a matter of replacement.