Do I have to replace a california emissions cat and O2 sensors on a 1998 bmw 318ti with the same thing?

Most likely you will need a California cat and O2 sensors. It is not unusual for the material inside the catalytic convertor to break down; there may even be a recall on them as there was on certain Jeep vehicles in the 1990s. Aftermarket cats do not contain as much of the platinum catalyst as the OEM cats, this is why they are cheaper, but may also be why you are having problems. However, your codes don't directly relate to the readout (rich/lean mixture) from the O2 sensors, so it is possible a wire has been burned or shorted causing them not to read properly. Have this checked by a mechanic.