I know there's two oxygen sensors, the upper and lower

Hello,

There are two upstream (upper) oxygen sensors. One in each exhaust manifold. They could be little difficult to see due to several components that can block the view.

I have attached diagrams for your reference.

Please let me know if you have any questions.
Thank you.

https://www.2carpros.com/articles/how-to-replace-an-oxygen-sensor