I'll try to explain why there are two codes.
Also I will try to explain the differences between the A/F sensor and the O2 sensor. Both of these sensors measure oxygen levels in the exhaust system, but they measure the gasses at different locations and for different purposes. The A/F (air/fuel) sensor is located in the exhaust manifold ahead of the primary cat converter. The sensor must be located in front of the cat, because the cat reaction changes the composition of the exhaust gasses, and the A/F sensor would not get the correct reading. It's job is to send a signal back to the ECU. The ECU looks at this signal as well as many others, such as MAF, throttle position, engine load, transmission TCM, air and engine temps, cam position, etc. From all this data the ECM adjusts the pulse width if the injectors, which controls the fuel air mixture for the engine.
Look at the attachment for more details on this system, called the Closed Loop operation.
If the signal from the A/F sensor is bad, it affects the operation of the closed loop system, so you get code 1148. Several problems could give the 1148 code. In this case the A/F sensor is at fault, so you also get code 1276.
The O2 sensor is located behind the primary cat in the exhaust system. (in the B pipes) It's primary job is to check the efficiency of the cat. It normally does not help control the ECU's function in adjusting the A/F ratio. If you have a problem with the cats, or the O2 sensors, you will get a 420 or 430 code.