In which cases do you say "oxide" when naming a molecule?
The naming nomenclature for inorganic molecules is confusing, but if you are studying chemistry then you just have to grit your teeth and learn.

First there are household names, common names and chemical names. Vinegar, acetic acid, and ethanoic acid are talking about the same molecule. We say water, but aqueous solutions. $\ce{SiO2}$ is silicon dioxide to a chemist, but quartz to a geologist. The list could go on and on...

In general an oxide is compound which has just some element, X, and oxygen in some combination - ${\text{X}_m\text{O}_n}$

Some anion combinations get a name. 


$\ce{CO3^{2-}}$ is carbonate
$\ce{SiO3^{2-}}$ is metasilicate
$\ce{CrO4^{2-}}$ is chromate