Could my upstream O2 sensor be causing a misfire on cylinder 1 and 3?

Hello, The O2 sensor sniffs the exhaust for the amount of oxygen remaining and tells the engine control module (ECM or PCM) that value. Now when the PCM gets the information, it starts to add or cut fuel, because only the driver can decide how much air the engine gets (how much you press the gas pedal). So, if it senses too little oxygen, the engine will restrict fuel. That, however, has a systemic effect, so all cylinders would have restricted fuel, not just the 2 cylinders on one side of the engine (1 and 3). Based on that logic, no, would have a P0300 code, not a P0301 and P0303. Knowing the engine in your vehicle, the very first thing I would do is compression check that side of the engine. They tend to eat head gaskets. Bad situation, but just reply to this thread as often as you need for assistance. You also might just have a leaking intake manifold gasket on that side letting in too much air.