I have a 01 grand am just had the head gaskets replaced and now when you start the car in park or neutral it will idle up and down for a little bit then cut off but drive or reverse it will not,and if you pull the pcv valve out while in park it will stop idling up and down. already replaced the pcv valve,throttle sensor. there is a ob|| code reading. could it just be my o2 sensor or the cat

Welcome to AF !

With out reading the code hard to say what it is.
Though idling up and down could be a vacum leak(s) or your IAC is clogged/dirty.
Here is a procedure to test your IAC, but a good cleaning of it and the TB usualy clears up the idle. :thumbsup:
Idle Air Control(IAC) Testing Replacing for 99+ (http://www.automotiveforums.com/vbulletin/showthread.php?t=663849)

Hope that helps !

try disconnecting your O2 sensor and see if it still does it.