There are only five oceans. They are the Pacific, Atlantic, Indian, Southern (Antarctic) and Arctic Oceans. You can see them on the map. Seas are usually parts of oceans washing continents. For example, Korea is washed by the Yellow Sea and the Sea of Japan. They are both parts of the Pacific Ocean.

I would say the terms could be used interchangeably, but Seas (Black Sea, Dead Sea, Irish Sea, Red Sea, etc.) are generally smaller than Oceans and sometimes are enclosed by a land mass.

There isn't really a difference between them, they're synonyms.

I would say that native speakers do not understand the difference between them, either. They both have a wide range of overlapping meanings. This is where a native speaker consults a dictionary. Try it! But in this case, you will not get a clear answer. They are basically synonyms and which one is used is based on tradition, custom, or personal preference. Large bodies of salt water, as geographic features, are named "ocean" or "sea." The name is what it is, and you can't change it. The Indian Ocean is the Indian Ocean; you cannot call it the Indian Sea. The Timor Sea is the Timor Sea; you cannot call it the Timor Ocean. In the case of geography, oceans are bigger than seas.

An ocean is generally large. A sea can be large, but not necessarily. Particular seas and oceans have names, so the Atlantic Ocean is never the Atlantic Sea etc. Other than this, they are essentially synonyms.