oversea和overseas的含義、詞性和用法都不同。oversea是形容詞，“海外的”或“外地的”，通常放在名詞前面；overseas則是副詞，“在海外”，通常放在動詞、動詞詞組或verb to be后面。

He qualified in London as a teacher of English overseas.

他在倫敦取得了海外英語教師的資格。

It's worth taking John along because he always goes over big with overseas clients.

帶約翰一起去值得，因為他在海外總是很走紅。

I'll take a three-month overseas contract, and my firm will hold my job open for me when I come back.

我要接受在海外工作3個月的合同，回來時候公司是會保留我的工作的。

He lived overseas for many years.

他在海外生活了多年。

A lot hinges on how we are viewed by overseas investors.

很多事都取決于海外投資者對我們的看法。