5. As shown in Figure 2, in $\triangle A B C$, $\angle C=$ $90^{\circ}, B C=a, C A=b$, $A B=c$, the two angle bisectors $A E$ and $B F$ intersect at point 0. Then $\frac{S_{\triangle O E F}}{S_{\triangle O A B}}=(\quad)$.
(A) $\frac{a+b-c}{a+b+c}$
(B) $\frac{a-b+c}{a+b+c}$
(C) $\frac{-a+b+c}{a+b+c}$ 
(D) $\frac{2 c-a-b}{a+b+c}$