O2 Sensors + are they the same?

[Lacoste82]

Hey all, im gonna order a new O2 Sensor, looked at a Denso one, is this an okey brand?

 

Also, it says on the site where im gonna buy it, if its okey that is, that the sensor is the rear one, as in the one mounted after the cat - But isnt the 2 sensors the same? reason im asking is, that if im not mistaken, the sensor that need to be replaced, is the one thats mounted before the cat, as the other one is "just" reading for exhaust values/pollution...!?

 

PLz correct me if im wrong:)

 

Thx alot, regards Mikkel