Here are some examples:
Not capitalized:
* We are planning a trip overseas.
* The company has offices overseas.
Capitalized:
* She moved to Overseas Bank. (Overseas is part of the bank's name)
* Overseas, the weather is much warmer. (Overseas is at the beginning of the sentence)
It's important to note that some style guides might have different rules, so it's always a good idea to check the specific guide you are following.