What non-K-Pop fans think:
The Korean word used by younger females to address older males.
In K-Pop context:
It is seen as cute when fangirls call their idols 'oppa' and some would even mask their real age in other to be 'younger' than their favourite artistes. It is not hard to see on social-networking sites or comments in Youtube where fans referred their idols as xxx-oppa.
Donghae and Eunhyuk from Super Junior released a single named Oppa, Oppa and they referred both themselves and Super Junior as oppas. The lyrics also made an appeal to the fans to call them oppa as well.