I bought some new o-rings for my kegs on Amazon last year and had to return them. The thickness of the rubber was less than the original, so it leaked. If you have the old cracked o-ring still, put them side-by-side to compare.
I didn't see where the OP said it was leaking on the post. Is it at the top where the poppet is supposed to seal, or at the base where the post is connected to the keg? Either way there's no way this keg could be holding pressure so it's going to leak gas when it's upright and liquid when it's on its side. The liquid should give you a better clue where the leak is or you could use soap solution to visualize the leak as gas.