To determine the complexity of the given expression, we need to analyze the number of operations it performs.

The expression is: $5n + 10n^2 = O(n^2)$

In this expression, there are two terms: $5n$ and $10n^2$. The term with the highest power of $n$ dominates the complexity, which in this case is $10n^2$.

Since the highest power of $n$ is $n^2$, the complexity of the expression is $O(n^2)$.

Therefore, the statement is **true**.