For a compact simple Lie group $G$, I here use the word flag manifold to mean $G/T$, where $T$ is a maximal torus. The $KO$-theory of the flag manifolds has been calculated by several authors such as Kishimoto, Kono, Ohsita, Yagita, and Zibrowius. The last two use some algebraic geometry and i heard recently Zibowius showed a type-independent method. I shall talk about a way to compute it in a manner of algebraic topology, especially for exceptional group cases.