Here's another way. It's a little less mathematical, but you still arrive at the final answer.
We know the dissociation constant for oxalic acid to be 5.9e-12.
So we can write
H2C2O4 --> HC2O4- + H+
We want to get the final equation to look like
H2C2O4 + OH- --> HC2O4- + H2O
Well, we can actually add another equation to the first one:
H2C2O4 --> HC2O4- + H+ K1 = 5.9e-12
OH- + H+ --> H2O K2 = 1.0e14 (it's e14 because we've reversed the water dissociation reaction)
If we add these two equations, we get the desired reaction. To find Keq, we simply multiply K1 and K2.
K1*K2 = (5.9e-12)(1.0e14) = 5.9e12
Hope this helps.