Idle fixed & How DOES the 02 work?

[randrews@post.cis.smu.edu (Robert Paul Andrews)]

Well, I finally fixed my idle/starting problem.  Big crack in the
intake boot.  100 bucks and 1 hour later my car idles fine and starts
everytime no problem.  NOW, here is the real question.  I recently changed
my o2 sensor thinking my idle problem might go away and I would get
my great gas mileage back (it dropped to 22 on the hwy.)  After I changed
it (still had the bad boot on) It slowly went up to 24.2 on the  hwy.  
Now, I then changed the boot and it has gone back down again.  DOES that
thing need time to learn the correct fuel mixture.  I am pretty sure
there is something that learns the O2 value and adjusts the fuel micture
better but not sure.  Now that I have changed my boot, before hand, would
my car have been running rich or lean, and now does the O2 sensor have 
to adjust it the other way?  I don't really know what is gonna happen
but I hope I will get my gas mileage back up.
Rob
randrews@post.smu.edu
Thanks in advance.