It isnt necessarily that you will only need o2 sensors if you have cat...no relationship at all..
[SIZE=2]The purpose of CATALYTIC CONVERTERS, esp the TWC(Three-Way catalytic-converter composed of precious metals such as Palladium, Platinum, and Rhodium) is to convert the unburned hydrocarbon (HC), carbon monoxide (CO) and oxides of nitrogen (NOx) which result from the combustion of gasoline. When the catalyst is working properly, the unburned hydrocarbon and carbon monoxide are oxidized by combining with oxygen. This oxidation forms water vapor (H2O) and carbon dioxide (CO2). The oxides of nitrogen are reduced to nitrogen and oxygen. The HO2s that you see before(pre-catalyst) and after (post catalyst), are just mainly to tell that the TWC is still working good. the smaller the difference between the readings of the precatalyst HO2S and postcatalyst HO2S means that the TWC is doing tis job well. [/SIZE]
Observe that sometimes in the morning when you start your car, water comes out from your exhaust pipe, TWC is responsible for that.