Decinormal means 0.1 N. The equivalent weight of oxalic acid dihydrate ($(COOH)_2 \cdot 2H_2O$) is its molecular weight (126) divided by its basicity (2), which is 63. Weight required = (Normality × Equivalent Weight × Volume in L) = $0.1 \times 63 \times 0.1 = 0.63$ grams.