Concentrating Oxalic Acid by Evaporation
Depends on how hot the plate is I'd say. Oxalic acid is hygroscopic, so it will be hard to completely dry it by just subjecting it to heat as you might just cause it to break down.

There's a paper and procedure on Organic Synthesis on how to obtain the anhydrous form, so you should check that out to see if that meets your requirements.