[govibe]

For this problem "Determine the empirical formula for a compound that has the following composition by mass: 11.66 grams iron and 5.01 grams oxygen"

I first convert the grams of Oxygen to moles... However, I don't know if I should calculate the molar mass of just "O" or "O2" (since oxygen is diatomic)... Same for other diatomic atoms if that were the case (BRINCLHOF)

[Yggdrasil]

Oxygen (and the other BrINClHOF atoms) are diatomic only when you're talking about their elemental form.  When these molecules form compounds, they are not necessarily diatomic.  So, to find the number of oxygen atoms in your compound, you would divide by the molar mass of an oxygen atom (16g/mol) and not the molar mass of an oxygen molecule (32g/mol).