The terms used to refer to siblings in Korean are:
동생 (dongsaeng): younger sibling
언니 (eonni): a female's older sister
오빠 (oppa): a female's older brother
누나 (nuna): a male's older sister
형 (hyeong): a male's older brother
Korean has a slightly more descriptive system for referring to siblings than Japanese or Chinese, though younger siblings are often referred to the same way regardless of their gender. Attaching honorifics (님 (nim): highly respectful,씨 (ssi): somewhat distant or slangy, or 놈 (nom): extremely disrespectful) is not necessary in most cases (and considered very strange if used in reference to a younger sibling).
With younger siblings, a gender-specific prefix is sometimes added (여 (yeo): female, or 남 (nam): male) to remove gender ambiguity. Though it is possible to add other specificity to either older or younger siblings' titles, it's more common to add their name to the title instead. Sometimes nicknames will also be used with sibling terminology in this way, though a younger person addressing their senior by nickname (even with title) implies unusual closeness.
It's more common to use the older sibling terminology for seniors and names for juniors, but a senior addressing a junior by title isn't unheard of.
These terms are used to address non-family more often than Japanese, often replacing upperclassman-underclassman vocabulary (though this is still used in some cases). PSY's "Gangnam Style", a Korean song that gained international popularity in 2012, uses oppa this way exclusively, identifying the target of the lyrics as women or a woman younger than the singer.