If you grab in the common location you can use a sensor to detect if you have the O2. This level is supposed to teach you that the sensor plate doesn't have to be in the same place as the sensor command. For example:

However, you don't really need to use sensors to solve this level. Remember that the output can have many atoms loaded on it and it will output them one at a time.

Just assume the input is an O2 molecule and place it in the ψ output and unbond it fully before outputting it.

This is the simplest type of reactor in the game, it just performs one task back and forth.