Probably not an O2 sensor but it could be injectors. If the misfire codes are always on those two cylinders, then you could just swap the injectors to see if the misfire codes follow the injectors or not. If it’s the injectors, there are places that will clean them properly and do flow bench testing before/after.
Cylinders 1 and 3 are next to one another. The misfires could arise from a head gasket failure between the cylinders.
Or both cylinders have suffered some valve/valve train issue.
Or it could be an injector problem.
Or low fuel pressure and 1 and 3 misfire first.
As much as I dislike messing with a car's fuel system in this case I think you are at least going to have to swap injectors moving the injectors from 1 and 3 to other cylinders and moving the injectors from those other cylinders to 1 and 3.
You don't have to do both at the same time. Try just swapping injectors between 1 and another cylinder. If the miss follows the injector then you can also proceed to cylinder 3. If the miss doesn't follow the #1 injector that is not a guarantee this will be the case for #3 injector. So you would or should try swapping #3 injector with an injector from an apparently healthy cylinder.
If the injector swapping doesn't help then that suggests the problem is internal to the engine. For a suspected head gasket failure or any issues with the valves a compression test is called for. And after maybe a leak down test.
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