That tube on the side of the cat comes from the smog pump on the right front corner of the engine.
The O2 sensor is located in the manifold BEFORE the cat.
As for those $80 cat's, don't waste your money. I had one last 2 years. Blew the guts right out of it.
You can't buy a used cat from a junk yard. It's against Fed. law.
+1, the tube just injects air into the exhaust to finish burning unbernt fuel in the exhaust. Just plug the tube, of remove the smog pump and get a shorter belt.