How to name this cyclic compound with an O in it?
Well, the molecule you draw is tetrahydrofuran, aka THF, but of course the only way you can guess that is if you already know the molecule. It's a very common organic solvent.

If you didn't know its common name, you could try to name it according to IUPAC nomenclature. It's an ether, and it's cyclic, so it can be named as oxacyclopentane. But IUPAC has specific names for cyclic ethers, which in this case is oxolane:



(from this organic chemistry book).