Oneegyaru literally translates to older sister gyaru, this is a fashion trend that focuses on looking more mature and older sisterly like but without looking older in age! If you are over the age of 20 and under 35 you would most likely tend to stick to this trend.
A classier style, very similar to regular gyaru-kei in fashion, but including only sexy or very stylish clothes, so for example you would wear a camisole instead of a tshirt, and heels are really the only footwear. There is a HUGE emphasis on designers brands, this is really the main point of Oneegal. Brands are very, very important as well as social status, which go hand in hand really. Don't do this unless you have plenty of money!