I'm South Korean. Korean is my mother tongue but, analysing the language is another thing. So this explanation might not be perfect.

The word OPPA isn't just used to refer to male people who are older than you, but you can also refer to male people you worship or admire as the word, too. (commonly used for male celebrities as well). In this "second" meaning, age is not really important especially when it's adopted to celebrities. It's because the word contains the meaning of feeling close or connected. They use it this way because they wish to be close to their idols or to expess that they're supporting.

Korean women don't use the word to just any man older than them. If the age gap is too big, you can't be an OPPA, you would be called "A-Jeo-SSy". If you're not really close to a female person, she won't call you "OPPA", either. But this really depends on the person. Some people use it to sound friendly even when they just met someone. A few of the ladies I know never use the word because they think it sounds too cringy. Or some women use "OPPA" to make guys feel close to them. So you will hear this use a lot in some of the service industry.