What is the difference between oxide and oxate?

Oxide is a monatomic (single-atom) ion consisting of an oxygen atom with a 2- charge (O2-). An oxide can also be just about any compound that contains oxygen in the 2- oxidation state.

Oxalate is a polyatomic (multiple atom) ion consisting of 2 carbon atoms and 4 oxygen atoms, also having a total charge of 2- (C2O42-). It is derived from oxalic acid, H2C2O4.