So I went under my car to change an 02 sensor and noticed a big hole in what appears to be the transmission. I can see a gear when looking into it.

I checked my tranny dipstick and it still has fluid but it seems to be the transmission I am looking at.....

 

You are looking at the flywheel and torque converter. That is the bell housing, and there is no oil inside there. There most likely was a rubber plug in that hole.

 

That's just a view port into the flywheel/torque converter.

 

That is the access hole for the bolts that attach the torque converter to the flex plate. Nothing to worry about

 

You wouldn't be driving around with no transmission fluid.

At best, you would be driving from right over here to right over there . . .

 

We don't all come out of the womb knowing that what's depicted in your photo is an axcess port allowing for the torque converter of the transmission to be unbolted from the flex-plate of the engine, or that the gear teeth we see in the hole are those that the electric starter motor engages to turn the engine over when the car is started.

We all had to learn this stuff, OP, so kudos to you for your quest for knowldge.

Mine has a rubber plug in that hole, so I assume they all do or did, but I don' think you'll hurt anything by not having one on yours.