oversea和overseas的含义、词性和用法都不同。oversea是形容词，“海外的”或“外地的”，通常放在名词前面；overseas则是副词，“在海外”，通常放在动词、动词词组或verb to be后面。
He qualified in London as a teacher of English overseas.
他在伦敦取得了海外英语教师的资格。
It's worth taking John along because he always goes over big with overseas clients.
带约翰一起去值得，因为他在海外总是很走红。
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个月的合同，回来时候公司是会保留我的工作的。
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.
很多事都取决于海外投资者对我们的看法。