Generally speaking, the glasses frame shape should be the opposite of your face shape in order to complement and make your natural shape softer. So, as you have an oblong face, square sunglasses with wide frames are fairly tall look best. This is the shape of the sunglasses, but you still pay attention to the color. It also should be suit for your skin tone. Ask your friends, strangers or sales person for their suggestions.
Generally, oblong face is suit for a variety of frames. It should be matching any style of frame, but not the straight line, that is to say do not pick the too high or too flat frames .The primary consideration is that what character you are. My friend is also oblong face, personality is very quiet, she have picked the little inverted triangle and oval glasses. We think she looks gentle and it is suitable for her. Sum up, as for the frame you choose what style, it depends on your own preference.