If you are talking about a specific ocean and the reader/hearer knows exactly which ocean you are referring to, use the first sentence.
The ocean is home to the majority of species living on our planet.
Again, if you are talking about a specific group of oceans (multiple oceans which are already mentioned) and they are known to the reader/hearer, use the second one.
The oceans are home to the majority of species living on our planet.
You can use use the third sentence when you you’re not necessarily referring to a specific ocean and you are talking generally.
Oceans are home to the majority of species living on our planet.
In your case, since you say that the line stands at the beginning of an article, it seems that you are talking generally about the home of the majority of species, so the third sentence would be preferred.