How many moles of oxygen are in 8.25g of O?

8.25 grams of O2 has 0.258 moles of Oxygen. Here's how you figure that out:

Oxygen is one of the 7 diatomic elements, so it is normally O2.

You find it's molar mass (or mass for one mole of it) by adding up two of the masses from the Periodic Table.