8

According to my dictionary they both mean "ocean", but I guess there is a reason for both words to exist. Am I right ?

5

海洋 is simply a technical/scientific version of 海. In scientific contexts, 大洋 specifically refers to the five global oceans on the earth, namely 太平洋 (The Pacific Ocean), 大西洋 (The Atlantic Ocean), インド洋 (The Indian Ocean), 北極海 (The Arctic Ocean), and 南極海 (The Southern Ocean). Imagine 大洋 is a oceanic version of 大陸 (continent). See this Wikipedia article.

Outside scientific contexts, 大洋 is not a common word. It's possible to use it simply in the sense of "big ocean", but I think 大海 is much more commonly used for this purpose.

2

They both mean "ocean" indeed but because of 大, 大洋 makes a listener think of big vast ocean, whereas 海洋 covers all kinds of seas big and small, such as 海洋生物 (marine life) or 海洋資源 (marine resources)

1

Basically agrees with Naruto’s answer. Here is an explanation from kotobank.jp:

海洋 かいよう ocean

地球表面の陸地以外の水をたたえた凹所で,水は塩分の組成が一定の海水である。海洋の総面積は約 3.6億km²で,陸地の約 2.4倍。海洋は大洋と付属海に分けられる。大洋は面積が広く,独立の海流系をもつ海で,太平洋,大西洋,インド洋の三つに分けることが多い。付属海は大洋の海流の影響を受ける海で,浅くて独立の海流系がない。付属海は地中海と縁海に二分される。

Therefore 海洋 means ocean, as opposed to land, and it includes 大洋 (Pacific, Atlantic, etc.) and 付属海 (adjacent sea).

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