Difference between $F(n)=O(n)$ and $F(n)\le O(n)$?
The notations $f(n) = O(n)$ and $f(n) \leq O(n)$ mean the same thing. Sometimes want to emphasize that the asymptotic estimate is coarse, and then you can use $\leq$, for example $\log n \leq O(n)$. This is not too common if all you're doing is just stating a single estimate, but is more common in a long calculation in which some steps are equations and some steps are inequalities, for example
$$ H_n + 5n = \log n + O(1) + O(n) \leq O(n). $$
In this particular example a simple equality would be more common, but in more complicated situations you could see an inequality. It is up to the authors' personal style. As a reader, you can just ignore this distinction.