Oxalic acid solution in a dry bottle?
Any chemical must be stored in a clean bottle in order to avoid contamination. With clean I mean without dust, and without traces of chemicals. When non-aqueous chemicals or solid chemicals are stored, then water also is a contamination.
Some chemicals are extra difficult on storage. They pick up oxygen, carbon dioxide or water from the air very easily and these must be stored in very tightly closed containers, otherwise they will deteriorate quicky.
Oxalic acid exists in two forms. The anhydrous form, H2C2O4, and the dihydrate, H2C2O4.2H2O. The anhydrous form easily picks up water from the air, forming the dihydrate.