Hi Natasha,
The formal charge of both the atoms in the hydroxyl radical is zero.
However, this question is asking for the oxidation number, which does not necessarily equal the formal charge of each atom (but they should add up to the total charge of the molecule).
To calculate oxidation number, you imagine breaking a bond between two atoms, and having the electrons go to the more electronegative atom, in this case the oxygen.
From this point, the charge on the atoms is the oxidation number. For this case, because the O has 7 e- after splitting in this way, its oxidation number is -1. As the H has 0 electrons, its oxidation number is +1.
I have attached a diagram of this process.
https://alumniubcca-my.s...keEPxvtrmAMRqZg?e=0aOrAjPlease let me know if you need more clarification.
Katt