You can say '오빠(oppa)' when you call your older brother, male friends/cousins/boyfriend older than you. It's not common that a girl calls a male friend older than her '형(hyeong)'. No boys will like to be called it because it will be highly likely to mean they aren't attractive to the girl. Some people would consider her a bit aggressive.

"hyung" is used when a boy calling a boy that older than him. "oppa" is used by a girl to call a boy that older than her.

oppa - use if the speaker is female hyung - use if the speaker is male oppa and hyung is like "kuya" in tagalog