joe and bob said it the best i guess...
just keep on killing things... most things give proper zeny anyway... some might give more than others (porings, saplings, peco-peco, whispers, wild rose etc) but it doesn't mean you have to hunt just them... just walk around killing things in rune-midgard and you're bound to make money...
about OCAs, i guess i'm a betting man, i like to open my OCAs... well the new SVN does take GR cards out of those, but i still think you should open them up... i have no doubt he made loads of cash selling his OCAs, but to me 2.5-3m is not worth the fun/suspense/dread of opening/having-opened OCAs...
-w-