Concentrating sulfuric acid
Calcium oxalate solubility depends on acidity, as the weak oxalic acid is protonated in highly acidic sulphuric acid.

Additionally, oxalic acid would be dehydrated if sulphuric acid was concentrated enough.

If we for convenience mark oxalate anion $\ce{(COO)2^2-}$ as $\ce{Ox^2-}$, then

$$\ce{CaOx &lt;&lt;=&gt; Ca^2+ + Ox^2-}$$

$$\ce{Ox^2- + H+ &lt;=&gt;&gt; HOx^-}$$

$$\ce{ HOx^- + H+ &lt;=&gt;&gt; H2Ox}$$

$$\ce{  H2Ox -&gt;[H2SO4] H2O + CO ^ + CO2 ^}$$

Therefore oxalic acid has no chance to create concentrated sulphuric acid.