Oval is what the world views as 'perfect'; symmetry as well. But for some reason, everyone is made in their own wonderful way, but for some reason....we still want to be 'perfect'.
Oblong transformed into oval - the object is to minimize height and add width. Styles such as the wedge and graduated bob haircuts add width in all the right places. When the hair hits at the chin or shoulder, it also helps to take away from the longer length. Straight across bangs with a side part also helps to minimize the length of the face. When you have wavy or curly hair, layers help to soften the face as well.
Avoid hairstyles that add height on top or at the crown. Middle parts will do the same, just like straight across, blunt bangs.