Affectionate term for one's own older brother, used mainly by children or within the family. Can also be used by a younger sibling addressing their older brother directly.
Big brother, let's play together!
Familiar, often vocative, way to address a young boy or a young man in a friendly or slightly patronizing manner. Not used for one's own brother in this sense.
My older brother is a high school student.
Hey, sonny, can you get the ball for me?
Combination of honorific prefix お, 兄 (older brother), and diminutive suffix ちゃん, expressing affection and familiarity.