Big-oh notations can be added, multiplied, and even exponentiated. But this is very confusing, what does it mean? The best way to think of it is as gnasher729 says: whenever you see $O(n)$ in an expression, you can replace that with some function $f(n)$ which is $O(n)$.
For example,
$$
O(n) \cdot O(n)
$$
means $f(n) \cdot g(n)$, where $f$ and $g$ are some unknown functions and $f \in O(n)$ and $g \in O(n)$. Similarly,
$$
2^{O(n)}
$$
means $2^{f(n)}$ where $f$ is some unknown function such that $f \in O(n)$, that is $f(n) \le C n$ for some constant $C$. And for a more complex example,
you can write something like
$$
O(n^2) - O(n) = O(n^2)
$$
What this true statement means is that, given some unknown functions $f$ and $g$ such that $f(n) \in O(n^2)$ and $g(n) \in O(n)$, $f(n) - g(n)$ is in $O(n^2)$, that is, $f(n) - g(n) \le C n^2$.