Big-O and little-o notation
Many people cringe at the notation $n^2=O(n^3)$, since, strictly speaking, $O(n^3)$ is a set of functions, defined by
$$
O(n^3)=\{f\mid \exists k&gt;0, N\ge 0\, [f(n)\le kn^3\text{ for all }n\ge N\}
$$
In other words,
$$
O(n^3)=\{1, n, n^2, n^2\log n, n^3, 3n^3+15n^2,\dotsc\}
$$
which is to say $O(n^3)$ consists of all functions $f(n)$ that are eventually less than a constant multiple of $n^3$.

With this understanding, to be precise we should write $n^2\in O(n^3)$, rather than the sloppier $n^2=O(n^3)$. This means that it is indeed notationally correct to ask whether $O(n^2)=o(n^3)$, since they are both sets of functions. 

[Note, by the way, I'm not asserting that $O(n^2)=o(n^3)$. I'll leave that to you to prove or disprove.]