inorganic chemistry - How does absence of nd orbital in Oxygen affect its valency?

The original question was 'Oxygen exhibits Oxidation state -2 to +2 but other elements of grp 16 exhibit only +2,+4,+6. Explain Why'. In the solution given i can't understand meaning of this line 'It doesn't have nd-orbitals due to which its valency doesn't increase'. Please explain how?

PS:I am a chemistry noob. So please provide as much details as possible :)