The other day my car, '83 Westy with a 1.9 L engine, started to run funny and hesitant on the freeway.  So, I changed the Oxigen sensor.  I was running without it and the car used a ton of gas.  Now I put a new sensor in there and it does not go the way it used to.  If I am on the freeway and I put my foot down, it would slow down and kind of choke.  Also at the speed limit it feels like it is lean of some fuel delivery is not right.  It does not have the power like before.  I can not tell if it is using more gas as used to, because I did not use up a full tank yet.  It seems OK for consumption.  
Any thoughts you guys have?  I am sure many of you went through the same.
I am eagerly awaiting your tips.
Zoltan