1998 Mazda Protege Trouble code help?

My friends Protege is up for inspection but she has a CEL so it has to be fixed first.

1998 Mazda Protege
1.5 Liter Automatic

My AutoXray CodeScout 2500 shows the following 3 codes:

P0421
P1195
P0505

Is the any one thing that will set all three or is there more than one problem here?

I know the P0421 is any O2 sensor but which one?

I think the O2 sensor issue is a problem by itself but can the other 2 be related?

Any help much appreciated.

1) Start with the upstream H02 sensor, it's the one closer to the exhaust manifold.

Doe's this car have a MAF, or MAP?
A bad MAF sensor canbe cleaned with CRC MAF sensor cleaner, but a MAP cannot.