When KMnO4 solution is added to oxalic acid solution, the decolourisation is slow in the beginning but becomes instantaneous after some time because

1.  CO2 is formed as the product

2.  Reaction is exothermic

3.  MnO4- catalyses the reaction

4.  Mn2+ acts as autocatalyst

When KMnO4 solution is added to oxalic acid solution, the decolourisation is siow in the beginning but becomes instantaneous after some time because Mn2+acts as autocatalyst.
 
Reduction half  MnO4- + 8H+ + 5e-  Mn2+ + 4H2O × 2
Oxidation half C2O42  2CO2 + 2e- × 5
Overall equation 2MnO4- + 16H+ + 5C2O42-  2Mn2+ + 10CO2 + 8H2O
End point of this reaction Colourless to light pink