Yes, there is a difference. Which is "Sea" in English, that is "Tenger" in Hungarian. However the "Ocean" is "Óceán". http://oceanservice.noaa.gov/facts/oceanorsea.html <- Read if you are interested. It's short and interesting. :-)
over can indicate direction, e.g. "the bird flew over the forest" would probably mean that the bird started in front of the forest, flew up so that it was higher than the trees, and when it got to the end of the forest, it flew down again. While "the bird flew above the forest" would mean that the bird was higher than the trees, flying around and around, but always with the trees beneath it.