In the given figure

(i) Let OA be x cm.

$\because \triangle O A B \sim \triangle O C D$

$\therefore \frac{O A}{O C}=\frac{A B}{C D}$

$\Rightarrow \frac{x}{3.5}=\frac{8}{5}$

$\Rightarrow x=\frac{8 \times 3.5}{5}=5.6$

Hence, OA = 5.6 cm

(ii)  Let OD be y cm

$\because \triangle O A B \sim \triangle O C D$

$\therefore \frac{A B}{C D}=\frac{O B}{O D}$

$\Rightarrow \frac{8}{5}=\frac{6.4}{y}$

$\Rightarrow y=\frac{6.4 \times 5}{8}=4$

Hence, DO = 4 cm