An oiler is a type of refueling vessel, which is used to transfer fuel from one ship to another. It is typically a small vessel, and is designed to carry fuel in its tanks, as well as other supplies such as food and water. The oiler is usually equipped with pumps and hoses, allowing it to transfer fuel quickly and efficiently. It is an important part of the maritime industry, as it allows ships to remain operational without having to return to port for refueling.