I wouldn't like to underestimate South Korea but I believe that they have a really challenging group in the World Cup. Even Ghana would put up a good fight against them I think. But let's say that they defeated Ghana, I still don't think that they can qualify for next stage of the tournament. Portugal and Uruguay are way stronger teams than them. Maybe we don't have the prime Portugal but they are still strong now. Even if they don't end the group stage as the leader, they would end as the second.
South Korea is a good, disciplined team that can beat Ghana, but it will be almost impossible to beat Uruguay with Cavani and Suarez or Portugal with Ronaldo.
Therefore, Ghana and South Korea will fight for the 3rd place, Ghana and South Korea have a chance to get out of the group only if Uruguay stumbles and plays a draw with one of them. Portugal is sure to beat both Korea and Ghana.