The atomic number of oxygen is 8, which means that it has eight electrons. The first shell of an atom can hold up to two electrons, and the second shell can hold up to eight electrons. Therefore, the first shell of oxygen is full, and the second shell has six electrons.

Here are the electronic configurations of the first and second shells of oxygen:

- First shell: 1s^2

- Second shell: 2s^2 2p^4