found this little guy hanging on for dear life

You really should try and get that check engine light off. If it weren't importantg it would be a "maybe consider checking the engine light.

 

I used a hex die to get the bolt nice n clean. The nut goes down perfectly.

Do I need to put a washer in there? I tried pulling up some pictures online but didn't find any that were clear.

At this point I'm thinking about replacing the O2 sensor itself. It look very easy.

 

There aren't any washers on the OEM setup, but their nuts have a flat bottom where they contact the sensor. The hard part about replacing the sensors is usually removing the old one. You have that beat. I would see if that one still works first. You'll know in about 100 miles if the MIL stays lit, and there are no other codes registering.