M, N, O, P and Q are five friends. Each of them has a different height. O is taller than P and N and shorter than M and Q only. Only one person is shorter than N. Who is the shortest?

Let the five positions from tallest to shortest be $1>2>3>4>5$.
* O is shorter than M and Q only $⟹$ O is at position $3$, and positions $1$ and $2$ are occupied by M and Q in some order.
* Since O is at position 3, the remaining positions $4$ and $5$ must belong to N and P.
* "Only one person is shorter than N" $⟹$ N must be at position $4$.
* Therefore, P must be at the remaining position $5$, making P the shortest among them.