Two functions are defined mutually: $E(n) = \begin{cases} 0 & \text{if } n = 0 \\ O(n-1) + 1 & \text{if } n > 0 \end{cases}$ and $O(n) = \begin{cases} 0 & \text{if } n = 0 \\ E(n-1) + 1 & \text{if } n > 0 \end{cases}$. Find $O(5)$.