The Wizard of Oz flag is cut in a X-division into four fields. There are two versions of the flag. As states in Chapter 17 in Dorthy and the Wizard of Oz the colours are pink, sky-blue, lavender, and white.
In Chapter 5 of The Magic of Oz the flag is composed of red, yellow, blue and purple.
The first image of an Oz flag manifested in Glinda of Oz in 1920. It consists of a green shaded octagon with 'O' and 'Z' superimposed upon each other.